I've been on Odyssey of the Seas and the Wish! The glass you see on the back of the ship are the windows of the Arendelle MDR. You can't really see in or out of them (even in the dining room). I agree with WaDiWo's statement that the Anthem (and therefore Odyssey) feels much more open and streamlined on the inside. You can walk from 270 all the way to the royal theater at the front of the ship. Passing by a majority of the specialty restaurants and bars along the way.
The Wish has many smaller pools with different depths compared to the Fantasy. Not all pools are always open at the same time. The size of the pool deck feels a bit larger with the layout. There are more public hot tubs on the Fantasy than the Wish. Even if the two hot tubs on the main pool deck of the Fantasy end up being kid soup haha.
The Hero Zone is an inside recreational space much like the SeaPlex but the Seaplex is utilized a bit better imo. SeaPlex will have bumper cars, roller skating, inflatable laser tag, corn hole, line dancing, volleyball, basketball, etc. The Hero Zone will have a couple of things going on like the Chip In Challenge or IncrediGames. I hope they'll utilize it more in the future.
I really enjoy both lines for different reasons. If I'm looking for a good ole fashioned, quality cruise I'll book RCCL. If I want to be immersed in the IPs I love with fantastic theming and service,
DCL is my splurge!