I would recommend waiting for the Treasure.
Second ships in a series often get tweaked to correct issues on the first one (nothing major - the ship build starts right after the first goes out).
While I expect the Menus in the Wish and Treasure to be similar or the same, the 7-night cruise will have 7 distinct nightly menus. Dining rooms with shows will not repeat that aspect, either. (No guest has to miss any menu or show if they eat in the MDR, thanks to their rotational dining and assigned table and time. You also should not have to miss the main show / entertainment in the Walt Disney Theater, as they build that venue large enough to handle the crowd in 2 seatings). There will be 7 nights of different entertainment on the longer cruise (15 night cruises get fresh menus and theater entertainment).
Other than repetition of menus, evening entertainment, Sail Away and Pirate Night parties, and daily activities, the biggest drawback is the turn-over day. Breakfast is rather rushed and limited. Guests are lined up, waiting for their group to be called to disembark.
Unlike some ports (San Juan, Barcelona, Vancouver, New Orleans), getting off the ship to do some sightseeing is not really an option. You are not within walking distance or public transportation of much that is compelling to see. Kennedy Space Center might benefit from more time than available.
Some guests utilize the pools or water rides onboard. Others enjoy drinking coffee and reading a book. The kids clubs will have Open House and registration after new guests board.
If you were older cruises, who just enjoy being on a ship, anywhere, these things might not matter. But with kids or being new, they seem to be a waste of a day.
The room stewards are also working in all the rooms to get them refreshed. The polite (requested) thing to do is to keep out of the cabin areas from 8:30 - 2ish. Maybe you are allowed in, but kindness would have you out of that area.