Been on Magic prior to its recent dry dock but here are some comparisons:
Magic:
Rooms are a bit dated, at the time I went on, the carpets were in need of replacement (Stains).
Mattress was on its way out but besides this and above, everything else worked as expected.
Rust was everywhere, even the paint job as a whole had an orange look to it (Pre-Covid).
Shower curtain, no big deal.
Staff was fantastic as well as all the dinning venues, older Carnival shows but you'll enjoy them.
Internet was abysmal.
No USB or additional outlets by the bed, just the desk area.
Celebration:
Blazing fast Internet.
Rooms are a bit more narrow but longer, bathroom is on the small side. It has a shower door and lots of storage space.
Plenty of outlets at the desk area, USB by the bed, both sides.
Newer ship, decor is more pleasing compared to Magic. Not that the Magic decor is horrible in any way, shape or form. More modern/upscale looking.
Lots of dinning options including some new restaurants (Shaq's Chicken, Rudi's Seafood Grill, ChiBang, Street Foods by Lido (3 options), Emeril's.
Lots to do on the ship while at sea (Bolt, Bigger Obstacle Course, Multiple Pools,Etc.)
Al Fresco options when it comes to dinning/eating, as well as plenty of outdoor seating areas.
Loft19 as well as an expanded Serenity area.
Ship is big and didn't feel crowded at all.
The only negative about the XL Class is the stage area where most shows are held, seating is limited as well as the viewing areas. It will be a turn off as you'll have to get there quite early if you want good seats.
Limited shops, for a ship this size, not many.
Also, depending on where your cabin is, the viewing angle to front/back of the ship might be limited. Unless that is, your cabin is one of the ones at the very edge of the ship or a Cove Balcony. Avoid a cabin next to a Suite (Corner Ones) as they can see into your cabin from their balcony. YouTube to get a better idea.
Main dinning room food or service was not great, however, I went on the inaugural cruise out of Miami. Expected as the crew is just getting their sea legs when it comes to service.
Breakfast is served in different venues, plenty of options, not just Buffet or Main Dinning Room.
Guy's Burger has its own covered seating area. Pig and Anchor is much larger, includes a "Heroes Tribute Lounge", plus you can get dinner as well as a free breakfast option here.
We had a great time on both, I prefer Celebration. In fact, we will be back on this Thanksgiving on a 7 Day to the Eastern Caribbean. With that said, you can't go wrong with either one.