A mini suite Reserve Class is not the same as a Retreat Sky Suite on X. Not really comparable to Aqua class either because with X you get your own private restaurant with their own galley. The Princess mini suite Reserve is not a suite. Princess does a disservice to their clients who think they’re getting a full suite. You will not receive any suite benefits as you would in a real suite.
The good news is that you will get a private portion of the MDR with the same menu and an added special course. You do not need a reservation and because the dining area is smaller you can expect a higher level of service. If you are comparing Reserve Class dining on Princess with Luminae, the Retreat restaurant you will be very disappointed. It is a night and day experience. Please do some research before booking.
Smoking is allowed in the casino, unlike Celebrity, if that’s an issue for you. Entertainment is about the same, probably more varied on Princess. No Martini Bar! No Eden! No Solarium! Princess does have The International Cafe open 24 hours for coffee, snacks and sandwiches. It’s terrific.
We love Celebrity, but we do enjoy some of Princess’s unique itineraries, so Princess and HAL are our next go to lines.
I won’t advise you one way or another, but you really need to do your homework to do a full and accurate comparison. You should also post this on the Celebrity boards. Princess cruisers are very loyal to the brand.