The ships are built the same, the differences are in theming.
Back of ship - Goofy hangs off the Magic, Donald off the Wonder
Lobby - Mariner Mickey statue on Magic, Ariel on Wonder.
Resturant - Luminere's on Magic, Triton's on Wonder
Entertainment District - Beat Steet on Magic, Route 66 on Wonder (and the names of the clubs are different as well).
Deck 9 Buffet - Topsiders on Magic, Beach Blanket Buffet on Wonder.
Also length of cruise - Magic does 7 day trips both Eastern and Western Caribbean, Wonder does 3 or 4 day trips to Nassau.
Some of the artwork is different as well. The feel of the ship is the same and both are wonderful. Enjoy your cruise!