Decor is different, both are beautiful. The nightclub area is very different in set up. The adult only areas are improved on the Fantasy, especially Satellite falls. Fantasy also has Bippity boppity Boutique. Animators Palate has animation night on Fantasy, not on Dream.
The differences between the 2 ships aren't huge, but I think they make a lot of sense, especially on the longer Fantasy cruises.
If all four ships offered the same itinerary, same prices my preference order would be:
1 - Fantasy
2 - Wonder
3 - Dream
4 - Magic
This is just my opinion, everyone has their favorites, these are mine. No matter what ship I'm on - I'm happy!