I have the carry-on and 24" medium Tumi Vapor (hard side).
Obviously, the 24" only came with four wheels, but I got the carry-on with two wheels...at first. I used both, and immediately changed the carry-on for the four wheeler. I personally value the maneuverability much more than the 1.5 inches of extra space. This is especially true if you are traveling with multiple bags. I don't think I can go back to two wheels.