A.S.
Hands down the Disney Cruise . . . that was one of the best trips we've ever taken as a family. It was incredibly relaxing (which is hard to believe with a ship full of kids).
Disney World is great, too, but it's a little more like basic training on steroids, especially if you go during the heat of the summer. We need a vacation from our vacation when we go there LOL.
The only thing I note about the cruise is your youngest - he/she might be too young for the kids' club (check the ages on their web site).
Good luck - hope you make the best choice for you!