On 234 we went with the swampthing-esk drive where you have all traction wheels, still lowered the center wheel 1/8" with the octagon style outside.
http://www.chiefdelphi.com/forums/sh...&highlight=234 We really like the "deflector shield" effect. A lot of robots like to hit other robots on the corners, but when you hit the cut-off corners the hits are kind of deflected to sides more than spinning the robot.
Downside: they are more complex to make and they weigh more.
That's just what I've seen. Like every other design there are pros and cons.