Your going to get 100 different answers from 100 different people for the same number of reasons. As far as which ship, food wise they are all basically the same. They'll all have kids meals with chicken nuggets, burgers etc on the menu. Theres pizza, burgers chicken all available for take out and room service also so the food issue is pretty much covered.
As far as which ship is best, you need to ask your self these questions. How long can you get away for, how much do you want to spend, and where do you want to go. For example, all of the Port Canaveral cruises, which are the Dream and Fantasy, stop at
Castaway Cay. That's the 3/4/7 night cruises. Occasionally the Magic or Wonder will come home and do trips out of PC also. Shell stop at CC also from there. Galveston, trips I don't think stop at CC. Miami trips do. The main differences in the Magic and Wonder, besides the Aquadunk is basically the décor. Everything is pretty much the same location, though it may have a different name. The differences between the Dream, and Fantasy, is the décor. Pretty much everything else with 1 or 2 minor differences is the same. Other then the names of clubs and restaurants.
All 4 ships have the same number of main dining rooms (3), and kids clubs which are pretty age segregated. Same number of theatres. There 2 on each ship. 1 will have stage shows, and the other will have first run Disney movies.
Personally I would start with a 4 day. A 4 day is just enough time to get your feet wet so to speak and not feel rushed.