My vote would be for the Contemporry Tower. More than likely they will be spending most time at MK and Epcot, and they will be able to use the monorail. Also if it is raining and they are not going to go out, they can get to the restaurants and such without having to go outside.
Problem with OKW is bus is required for eveything (unless driving).
Note that the rooms at the monorail resorts are larger than any other, with Boardwalk and Y&B next and the smallest deluxe rooms are AKL and WL.
Of course all the deluxe resorts have a 4 cubic foot refrigerator while the one bedroom and larger have a full-size refrigerator along with cooking facilities.