Don't forget though if you have a scoop you have 13.6kg + some of the weight of the other bot. In addition more torque will help in the event of a wheel becoming clamped to break the clamp or some such. And you still have acceleration to consider (though even drill motors are pretty good for that one). Other things to consider are matching the drive train to the weapons you're running. If I'm running an axe on 6 cell LiPos I'd like a drivetrain that can take 6 cells without large overvoltage.

EDIT: Interestingly I did experiment with this in the antweight class with a 4WD pusher using small pololu wheels. Moving from 1 wheel per motor (4 in total) to 2 (8 in total) was a massive improvement. But I tried 3 per motor (12 in all) and performance suffered, for that I found 2 wheels per motor was optimal.