We're thinking that a good mix of the two will be helpful. Since the object is to get and score balls, maneuverability is going to be important, and also if a robot is playing defense by blocking/pushing then that robot is not scoring. Then again, there will be times when you don't want to be pushed around, so having decent traction, but not so much that it makes you robot hard to steer, should be helpful.
In other words, it's a draw
Driving over a bump with a 6wd robot looks scary. Adding two more wheels might work better, but we haven't tried it yet so I dont know for sure.