We prefer the Disney cruise to the parks. You will be able to see characters, do activities, and see amazing Disney shows each night. Plus, you will be able to go to the beach/visit islands during the day. As folks have said, it's more fun and relaxing. And the food is much better on the ship than at the parks.
Ignore the poster who criticized the short cruises out of Miami. My two cents: piggyback a few nights prior to a 4 night cruise out of Miami. Rent a condo at the beach (sunny isles is nice, or head to the keys). Or do a couple nights at the parks before sailing out of port canaveral.