The ships are almost identical. I personally liked the decor slightly better on the Dream--art deco. The Fantasy would be better decor wise if you like the princess theme. The inside adult areas are different, I cannot say one was better, I did not like the center bar areas though. The sports bar and pub on both were nice. The adult upper decks and water areas and shaded areas on the Fantasy make a positive difference.
Overall, I Liked the Fantasy better, here is why:
Dream=short cruise, Fantasy=seven day cruise
Fantasy, way better water features and upper deck adult area. The Aqualab was a nice wet diversion for kids and adults.
Dream=slightly more stressed clientele due to shorter cruise, (slightly rowdier crowd too).
Fantasy has a better itinerary, more exotic locations. Nassau is blah.
Cannot go wrong with either. Decide on the best itinerary, and length of cruise for you.