but my team, a basically rookie team, had perfectly working treads last year, our problem was with bad bearings (they popped out whenever a robot hit us, and half the time when we hit a goal) and with gearing... for some reason no one, not any of our engineers (who we love) nor our teachers (mostly physics teachers too) could help us figure out the right gear ratio for the drive train - ended up being the fastest the chip motors could put out, provided the robot weighted nothing...
on topic: the treads normally take a little extra work, but if done right, can be hell on wheels, (i mean treads).
wheels are easier, but normally have less traction
bottom line
? either works, just get something with moderate traction and ok speed, and then tinker until its good, because as long as the robots well designed (over all), an ok drive train will still murder (i mean beat in a friendly manner) most other teams