The motors you choose for a mini-sumo robot will vary with the strategy you want to use.The trade-offs for competitions like these are typically speed vs. torque. If you want a robot that can out maneuver the opponent, speed might be a good choice. If you want to have a power house robot that can push other robots out of the ring with brute force, you might use motors with greater torque.
As an example, we generally recommend our 50:1 or 75:1 micro metal gearmotors for use in our Zumo Robots.
Good luck in your competitions. We would love to see what kind of robot you build!