The two ships are virtually identical. The "fancy" restaurant has different names (Triton vs. Luminere (SP?). The cabaret area at the front of the ship, deck 3 is pretty much layed out the same with different names. The layout of the ship is very close and if you know one ship you generally know the other. Teen hangout is different since the refit of the Magic.
A 3 or 4 day cruise on the Wonder is great (have done one of each within the last 2 years). The short time is good if you don't know whether you will like to cruise or not. Since the time is limited you see the three main Disney shows on either the 3 or 4 day cruise. The 7 day Magic (Jan of this year) is great with the same 3 main shows plus other "lighter" shows.
Either cruise you take will have the same Disney characters with photo/signature opportunities. Since the 7 day has more days the opportunities are generally repeated.
Food on both ships are equal in quality and quantity. Both pretty good.
The big difference between the two ships is the itinerary. The 3/4 day Wonder goes to the Bahamas and the private island (
Castaway Cay). The Magic 7 day goes Eastern one week and Western the next week. Western is Key West, Cozumel, Grand Cayman and Castaway Cay with 2 days at sea. The 7 night Eastern is St. Marteen, St. Thomas, Castaway Cay and 3 days at sea.