I can't say what WDW is like because we will be taking our first trip in August so I have nothing to go by personally but based on all the planning I've had to do and all that I've heard, WDW is more about getting up, getting out, riding rides, eating, rushing, buses, ADRs, fireworks, lines, crowds, etc - it seems to be a more quick paced, exciting type of fun.
I've been on 3 Disney cruises and have enjoyed them immensely. It is a slower pace and while it can get crowded around the pool on sea days it's not crazy packed like the parks (the Fantasy's total capacity is something like 3,000-4,000 including crew so there's plenty of space to move around). Plus there are so many things to do it's hard to get bored. There's a whole adults only section with all kinds of bars and clubs so if you wanted a grown up night, you can send the kids off to the kids club. But it works both ways - while the kids club is a great way for adults to have some grown up time knowing that their children are safe, it's also a way for kids to get a break from the parents as well, play some games, make new friends. We never used it but people we've met onboard said their kids love it. I like the rotational dining - you have the same table number at each restaurant each night and the same servers so you will always have a table to sit at for dinner and the servers get to know you and any special needs you and your family might have. But you don't have to eat in the restaurant if you don't want to, you can get room service or eat at the buffet and since most people are at dinner, the ship is yours to explore. I especially love formal night, people gets dressed up (but you don't have to) and the CMs pass around cocktails and you can meet the captain. The character meets/photo ops are nice too, thee are lines, but you may have a 30 - 45 minute wait max and that's really only for Mickey. Most waits are more like 15-20. There are a number of port adventures you can go on if you want to but those cost extra and if you don't go out on port days, the ship is virtually empty (on our third cruise we skipped Cayman because we had done it twice before and we had the entire pool and deck to ourselves, it was great). We saw Aladdin the musical before it got to Broadway and if you do the Caribbean cruise they have a pirate night with fireworks. It is pricey but all your food (except for Remy and Palo) and non alcoholic beverages are included.
So if what I'm thinking WDW will be like is true, comparing the two is like comparing apples and oranges. They both have their good points.
EDIT: I forgot to add that while there are scheduled character meets, they also just roam around the ship so you may just wind up riding the elevator with Belle or Taina or Chip and they will stop and talk to the kids and take photos.