How old are your kids?? Because if you're looking for a vacation where you have a vacation with and WITHOUT the kids, then the cruise is the way to go! On our
Disney cruise we could NOT get our five yr old to come out of the kid's club! We only ate one meal with her, and we forced her to do that one since it was Animator's Palate. She had such a good time on the cruise, and so did we. It was nice to get some adult time, but we also had family time (when we could coax her away!). However, if your kids are a lot older, the kid's club may not have the same affect on them.
Also, how often do you get to visit the theme parks? If you get to visit pretty often then I'd do the 7 night cruise. It seems that by the 3rd night you've just gotten to really know your way around the boat and then it's time to get off! I think 7 nights is the way to go for cruising. However, if you can't get to the theme parks easily, then I'd go for a land/sea package.
Either way, you'll have a blast!