Servos needn't be too slow - the TowerPro MG90s are easy to speed mod with some spacers, which increases the speed considerably to be on a par with some gearmotor driven robots.

They have nowhere near the power or torque or durability of the motors though - and as a pair of motors, mounts and wheels cost less than £40 I would reccommend them strongly if you're looking to be competitive (although with a big enough weapon, servo drive is doable!)

Ants do have a very busy calendar - there's three World Series a year and at least that number of other events, plus the fact they run at just about every heavyweight event too, I don't think any other weight class will get you more battles!