For a first cruise, I would focus more on the dates and itinerary, since the Disney ships, except the Singapore-based Adventure, are similar in their dinner structure (3 rotational dining rooms and 1 or 2 adult dining rooms with an additional cost), shows (3 main shows plus something in the theater most other nights), kids programming (drop-in areas for ages 3-10, 11-14, and 14-17, plus a nursery for those 3 and under), and lounges (some open to everyone, some adults-only in the evening, some adults-only all the time). The layout of the 3 newest ships (Wish, Treasure, and Destiny) is different from the first 4 ships, with the adult lounges spread out rather than one area.
That said, of the older ships (Magic, Wonder, Dream, and Fantasy), the Fantasy is my favorite, for the reasons greensombrero mentioned: the Midship Detective Agency is fun, and the adult area on the Fantasy is better themed than the similar area on the Dream. But I have happily cruised on the other ships when they are going somewhere I want to go.
Happy planning!