We've only been on the Wonder and the Dream, but I preferred the Dream by quite a bit. The Wonder was a lovely ship, but felt a little worn when we cruised (Jan 2012) and for some reason felt more crowded, even though it was bigger. The Oceaneer's Club was smaller than I expected and seemed a bit dark to me and the nursery was a bit disappointing as well. I also loved the Midship Detective Agency on the Dream. Animator's Palate was much better on the Dream and we didn't care for Parrot Cay. And the Nemo's Reef splash area had no comparison to that dinky little water play area on the Wonder. I really felt that for young families (our kids were 1 and 3 the first cruise, 3 and 4 the 2nd) the Dream had much more to offer. I loved the new ship so much I'm now trying to save up for a seven day on the Fantasy!
That being said, I've been on 7 cruises in my life, and the Dream and Wonder rank number 1 and 2 by a long margin, and I'd go on either again in a heartbeat!