I've been on the Dream once and will be on the Fantasy next month. The rest of my [embarassingly large number] of Disney cruises have been on the classic ships. There are some great features on the Dream/Fantasy, but I love the Magic so much. I love Diversions, I love the Cadillac lounge. I love Lumiere's (MUCH more than the Royal restaurant on the Dream). On the other hand, I love the hot tubs in Quiet Cove MUCH more than the classic ships' Quiet Cove. I like Cabana's more than Topsider's. And I have to admit, I really liked the mini-golf on the Dream. Oh, and I also liked the set-up for Shutters better on the Dream. Much easier to find my family's pictures.
As far as shipboard activities, I can't really speak for the newer ships. But it seems like there's always something to attend on the classic ships. I have to think that the available activities are similar on the Fantasy. The only thing I remember for sure on the Dream was the walking tour, which is also available on the Magic/Wonder.
To me, I now choose my DCL trips based on where I want to go and what's affordable at the time, rather than the ship. Plus, whichever ship is showing Golden Mickeys gets a slight edge. . .
If I had to make your decision, I would almost always choose 6 nights over 4 nights for the same price. UNLESS the longer itinerary didn't include Castaway Cay for some reason. Then I would have to seriously consider the shorter cruise. As much as I loved the Alaska cruise, there's just something really disappointing about disembarking after your Disney cruise and realizing you didn't have a day at Castaway Cay.
P.S.-- @
bradandsarabr, speaking of Castaway Cay, I love your avatar and hope to get one of my own next month!!