First the TL/DR - Would be very happy to cruise Carnival again, but not on that particular ship...
Now the longer thoughts...
First, this was only a short cruise on the Carnival Sunshine - 1 day in Bermuda out of Manhattan for 4 days total. I'll be using our last cruise on the Island as a baseline, as it's not fair to compare a smaller ship like the Sunshine to the Royal.
The Sunshine is an older ship, built in 1996 as the Destiny and refurbed and renamed the Sunshine in 2013.
Whether you would be happy with Carnival or not probably depends on your priorities. There are some things they do better (and I assume procedures are the same across most if not all Carnival ships) and there are some areas that don't measure up. Everything is subjective, of course.
First up, the ship.
Carnival seems to not mind design elements that end up with a lot of 'you can't get there from here' and bottlenecks. For example, the Galley is located in the middle of deck 3, which means to get from Guest services on deck 3 to the MDR aft you need to go up to deck 4 and over or down to 2 and over. And if the comedy club is in use on 4, that path is blocked. Guy fieri's burger bar and the resultant line overlaps with the buffet entrance. On deck 5 you have to pass through the casino, etc. (Side note, why is the arcade next to the casino - this has been changed on other carnival ships but just seems like a bad idea)
Decor wise however, it was pretty nice. A little bland in some areas (surprising, I really was expecting it to look like Mardi Gras had vomited) but honestly in the corridors and venues you could have told me I was on Princess and I probably wouldn't have argued.
Their comedy/small theatre lounge is in a bad location but is a much better facility than on the Island (smaller, but they run multiple shows to make up for it). It's more intimate and comfortable - in short its what Princess Live should have been. In general, they have more small venues that allow for much more programming flexibility and they use it.
Their main venue seemed small for the ship but never filled.
The atrium would not have been out of place on any Princess ship except for the large bar in the middle of the lowest deck. Since the MDRs are located aft and have a better system however you don't get dinner crowds clogging it up.
We had what they call a Spa Suite and honestly it was pretty small for a suite (call it a Mini and a half in Princess terms) but it did have a double balcony and full tub/shower plus access to the thermal spa (which was pretty much the same as the one on the Island in terms of amenities, except you had a window where the loungers are). We checked out a few cabins and they seemed overall a little tighter than Princess but manageable. Plenty of storage.
Design wise, they were cruise line beige. Which is fine. It's worth noting that not all rooms on Carnival have refrigerators.
For a standard itinerary most people would be fine with a comparable Carnival room. Might be a bit tight for something like a TA.
Dining - MDR
There are three aspects when discussing Dining, system, menu and preparation....
System - quite frankly Carnival kicks Princess's tail. Using a 2 level MDR plus a second satellite MDR they can mix and match traditional and anytime dining as needed. If you have anytime dining, there is a dedicated check in desk on deck 5 where you can make any requests. Even though you have anytime dining, the system knows which servers you have had and if you liked them will place you back in that area when possible so you get the best of both worlds in that respect. They also have a lot more 2 and 4 tops (no real sharing option) and they are not right on top of each other. TD diners cannot check in at the AD station, you must get approval from the MD to be seated. If there is a wait, which was only one night of the cruise and less than 10 mins at peak time, you get a pager which works anywhere on ship except the casino.
If there is a wait, the Alchemy bar has seating right there as does Ocean Plaza where they only play live music during the dinner hours, no activities, so plenty more seating. Once your pager buzzes (or if no wait) you don't need to go back to the desk, just to the entrance on deck 3 where you are typically seated in less than 5 mins.
Another nice touch is the Taste Bar. Located in the same area, it serves free samples of what is available in the other dining venues like the specialty restaurant etc. And at lunchtime it doubles as an out of the way lite meal venue (soup, salad, small wraps, etc).
Menu - This one is going to depend on what you prefer. Princess definitely skews more European than Carnival in their menu selections, less sauces on Carnival for example. Also Carnival has upcharges for certain menu items like filet (a flat-iron steak is the always available option). They do however each night have one item listed as 'Rare Finds' which might be something you have never tried, like Frog Legs.
Preparation - Honestly? I'll rank how our meals were prepared and presented up with almost any MDR on Princess. Over 4 days I can't recall any complaints with the food, at all. (There was a service issue, but it turned to a net positive, more on that in a bit). We ate all dinners and most breakfasts in the MDR. This was a big surprise to us, how well the food was done and presuming the type of food meets your liking, I don't think any Princess cruiser would be disappointed.
Dining - Other Venues
Carnival does have some upcharge venues (Steakhouse, Italian, Asian, Sushi, Shakes) - the interesting thing is that they use some of those to provide additional lunch options - the Asian restaurant becomes a (somewhat slow) made to order mongolian wok station, the italian venue becomes a pasta bar and these are no charge. Poolside you have a Guy Fieri burger bar and blue iguana tacos and burritos (never had a chance to eat at either of those). Add in the taste bar and buffet and you have a lot more lunch options than is typical on Princess.
The buffet was Small, but had more than enough to keep most people happy. Two main kiosks in the middle serve the same items and then there is a Deli kiosk (open longer hours), a pizza station (24 hours), 2 self serve beverage stations Juices in the AM, lemonade and iced tea rest of day plus coffee and water) and other stations that changed according to meal. While the space was small, Princess could learn something from their organization which kept things moving.
Room service is available 24 hours, no charge, limited menu.
Sea Day Brunch - now this is brilliant and something I think princess cruisers will love. On every sea day the MDR is open straight through from 8:00 AM to 1:00 PM with a brunch menu featuring everything from eggs to grilled salmon. This means you can sleep in and still get a full breakfast if you want.
Main Stage - the entertainment structure is very similar to that on Princess, a set of performers doing revue style shows. The two on this sailing were Studio 54 and Epic Rock (I think you can guess the themes). Costumes and staging were surprisingly good, probably a bit better than Princess, talent level about the same. The middle night was Hasbro the Game Show - a lot of fun and a great family show, later that day was the Love and Marriage show - a carnival staple, which we didn't see.
Lounge - each night the 4th floor lounge becomes Punchliners, where they bring in mid tier or better comedians. There were two on this cruise and each did a mixture of PG, R and Explicit shows, all with different material. Both were far funnier than any I have seen on Princess.
There are the usual musicians around the ship (mixed talent level). Again, if you were sitting in the atrium, eyes closed at any given time, no idea you were not on Princess.
Cruise Director - No joke, Emma was one of the best cruise directors I have seen at sea. Yes, she had a few vocal mannerisms that got a bit tiring at the end of the cruise, but generally she was always on and she was on and about a lot. Carnival splits the CD role into the onstage CD and the backstage Entertainment director and it really frees up the CD. If you like interacting with the CD this is a plus.
This is where I might be found a bit controversial. While I don't think the staff was as good as on Princess, I think the staff was better than on Princess. Wait, what?
What I mean by that is the staff did not seem as polished as Princess staff does generally. They were more casual, joking,, etc and I know some folks might not like that. That said, I felt far more of a general desire to do a good job than I did on Princess, the staff genuinely seemed to care about your experience. Officers were far more out and about and mingling with passengers and the casual interactions really worked.
Problems happen on cruise ships. The key is how those problems are handled. During a Sea Day brunch there was some kind of problem in the galley. Our server was obviously stressed and this ended up resulting in a service delay and my order being switched with one at another table, we both touched the food before we realized. Now, on Princess, I would normally expect the issue to be corrected, and it was. What I didn't expect was for the server to seek out the MD and explain the issue so that the headwaiter and MD both came over to apologize, explain the issue, etc. Plus, the FB director also sent a note to the room, with snacks. The culture was clearly one of teamwork and accepting that issues happen, and I saw other examples of that all over the ship. To be blunt, that is not the shipwide culture I have experienced on Princess in general, and I wish it would be (and yes, this is only one ship, management on another could be totally different).
Our cabin steward also offered something we are not used to on Princess. Once a day service. I want to stress this is OPTIONAL but great if you like to sleep in. Instead of early morning and evening turndown, the cabin would be done once, around lunch time and if you needed anything at other times you could call housekeeping. I thought this was pretty brilliant as you did not have the 9 AM tap on the door when sleeping in AND the steward had more flexibility in their day. Some people may see this as a downgrade in service, we did not. Our steward was also very personable.
Same with Dining staff - we ended up having the same waiters most dinners and even one breakfast. Now, they joked with one another in a way I would not expect on Princess but that never affected service (in fact, I think it enhanced it). (Side note, they work in teams of three over a larger area than on Princess - each section has a lead waiter, assistant waiter and junior waiter - it works well the way their DRs are set up)
This is one of those areas that if it bothers you on Princess, well this may not be the line for you. I will start by saying that the ship was CLEAN. Really clean. But there was definitely less attention to cosmetic maintenance. The railing on our balcony had a large section where the varnish was almost totally gone, and there were more rust spots than I am used to plus a lot of sea salt accumulation. In the MDR, a broken window shade was held in place by duct tape. There was wallpaper damage near the elevators. Not sure if it was just this ship (I think not because of the general attitude of staff) or Carnival has a less aggressive attitude overall to cosmetic maintenance.
If you put waterslides on top of the Island and have the adult pool uncovered and facing the main pool from 2 decks up, you pretty much have the Sunshine. Personally I was not a fan of that setup as the adult area except for a very small and crowded area really picked up the noise of the pool area (see Atmosphere, below). There was a definite lack of quieter spaces such as you might find on Princess and this could be a key factor (we just used our balcony and could not hear a thing). The adult area is free, there is no equivalent to the Sanctuary.
Kids clubs seemed pretty similar, we didn't go in, there was a small 24 hour playground as well for younger kids plus a splash zone. For older kids there was a 2 level ropes course, sport court, etc.
Shopping and Spa no difference at all between the two.
This is the big difference. Carnival is definitely more high energy and far more casual than Princess. Elegant night in the MDR means no shorts and t-shirts (shorts except cutoffs are permitted on casual nights). There was no formal night this cruise, people did dress up for the elegant night, others did not. Activities routinely ran till 1-2 AM and the pool deck was officially open till 2 but apparently never really closed.
The common areas all seemed to be a bit louder, more boisterous. Not obnoxious, just...more.
And alcohol is more prevalent. Not in your face, it was not the booze cruise we had been fearing, but bars are more of a structural focus than on Princess.
I should point out we never felt nickeled and dimed. In fact the only time we were approached to buy anything was on embarkation day, where someone walked around the buffet selling wine packages. We also never felt compelled to try specialty dining, the MDR was just fine. There is an all-you-can drink package for $50 per day pp, and a soda package for about $7.50 per day. Taking into account Carnival fares tend to be a bit lower than Princess on many itineraries it does make a good budget option without giving up too much.
Overall, if you look at a cruise ship as a place to eat, drink, and enjoy entertainment between ports, I think based on our experience you would be happy on a Carnival cruise. If however, you are looking for more of an upscale experience, you are probably going to be happier sticking with Princess.