I had the same question but asked my son. This is my understanding: Wide Screen is formatted to best replicate the way it appears in the movie theatre. You will most likely see that it doesn't fill up your whole TV screen, leaving some black at the top and bottom. Fullscreen has the aspect ratio adjusted to fill up your TV screen. Most film purists would most likely prefer Wide Screen.
If you have an LCD flat screen or are planning on getting one, the wide screen is the way to go. For full screen they crop the film to fit the 4:3 aspect ratio of the old normal TV's. Wide screen has an aspect ratio of 16:9 or even higher (2.35:1). There are some, like the 2.35:1 ratio movies, that will still have black bars at the top and bottom even on the HD LCD TV's..
I wondered about the difference too. Having just ordered 2 other movies, one in widescreen and one full screen, it became obvious immediately. I watch DVD's on my 15" laptop. Widescreen gave me picture from top to bottom and side to side. Full screen is 'black boxed' on both sides giving me a much smaller view. Now I will know to order widescreen to get a full picture to view.