We sailed The Dream for our 25th anniversary and was our first cruise ever. We LOVED it! I've sailed on her 4 times now and it's still as exciting as the first time! She is exactly like the Fantasy with the exception of the theme in the adult areas and the color scheme of the ship. The Fantasy has the beautiful peacock blue colors and the Dream has the nautical navy blue colors.
I liken it to the Beach Club Resort vs. the Yacht Club Resort at WDW. The passenger atmosphere is a bit different as has already been said. The Dream passengers mostly seem to be on a little bit of a faster pace because of the shorter length but my first step on the Fantasy I felt like I had just entered "island time"! Nassau isn't that great but it's a good day to stay on the ship and explore or eat lunch in one of the main dining rooms. Because it's a 4 day cruise you won't have time to do it all, but your advantage is that you've been on the other ships and can skip over all the things that are similar and just enjoy the relaxing time aboard and enjoy the beauty of the ship! I say book it!! Enjoy your cruise!!