That is a hard question to answer...
The Allure is a completely different experience. The ship is extraordinary! It's a much more of a "complete" family resort style cruise ship. The entertainment is top notch and thrilling. I could go on and on about how awesome that ship is.... If you read Jen's Allure trip report I think you'll get the idea.
Now with the Fantasy. Yes the ship was awesome! The pools were frankly pathetic in comparison to the Allure. But the Aquaduck is fun! As fun as the Flowrider? Well not to me and the older kids, but to Jen and the little one it was more fun! The entertainment is cute and very family friendly, not necessarily the most talented performers like on the Allure, but still fun... The camraderie on the Disney ships is what kind of makes it special to me. If you are going on a
Disney cruise, then you like Disney and Mickey and all that... That gives everyone something in common right off the bat... It's easier to hit it off with others, especially Dis-ers...
$2000 though...
To put it in this perspective, you can go on the Allure and still have enough left over for a Carnival Breeze 6 night cruise... That's about what we paid for our Breeze cruise for all 5 of us.
We'll still keep alternating between Disney and Allure/Oasis even though Disney is a little more $. Only about $1000 in our case. We just like the different experiences...
If you've been on a Disney Cruise already, I'd say try the new experience. If not, and you absolutely love Disney, I think
DCL would be worth the $. Or you can always use that extra $ to have a few days at WDW prior to your RCL cruise.
Ron