In my opinion, there are pros/cons for each of the ships.
The sail away party is different between the classic and newer ships - I prefer the party on the classic ships.
The newer ships have more technology - especially in the kids clubs (magic playfloors for example), more 'rooms' with varing themes, and a much larger teen area that includes outdoor space. If you don't have kids, then no issue. My kids like both, but do like the playfloor and the increased space in Vibe over the spaces in the classic ships. No midship detective agency on classic ships
The classic ships 'feel' less crowded to me. More ability to get to know more cast members and bump into folks you meet and chat with during different events, which I like. If going to
castaway cay, then less folks on the island.
Palo seems to vary a little between the ships (pizzas for example). Palo is always wonderful, but there was something about it on the classic ships I seemed to enjoy more.. . .
No Remy or aquaduck on the classic ships.
Longer cruise on the classic ships offer the 'tea' with Wendy or Alice and a character breakfast, which the newer ships do not.
I think all 4 are awesome and would not hesitate to go on any of them!
Enjoy!