We have done both and both are totally different experiences. Disney World has the rides and attractions and is what people think of and have in mind when they think about experiencing Disney - pictures in front of Cinderella's Castle, meeting the characters, etc. However, the Disney cruises are totally awesome too, and really, I think your girls would have a ton of fun either way. They do have the Kids Club on board the ship that you can drop them off at (if your younger child is potty-trained) if you and hubby want some adult alone time, and lots of other fun things that all of you can do together. It's not as hectic as running around Disney trying to see and do everything, waiting in line for rides. The meals are included. You get to stop one day at Castaway Cay, their private island in the Bahamas and hang out on the beach. They do have some characters on board that you can still meet, take pics with, etc. We did both almost a year ago, with stepsons ages 16 and 17, and daughter age 3.5, and we all had an amazing time, at both Disney World and on the cruise ship.
What you both need to do is research both and figure out together what you want to do. Since you have your heart set on Disney World, I would probably do that first, and then the girls can experience what Disney World is all about. But we found the cruise to be a phenomenal experience as well, and can't wait to do it again. Maybe you could consider doing the cruise at a later date. Or if you really want to go all out, and can afford it, do both - spend 4 or 5 days at Disney World, and then top it off with a 3-night or 4-night Disney cruise!