in terms of maneuverability, you can't beat a 2 wheel differential steered platform.
The 4-wheel versions are basically "Skid-Steer" like some construction equipment loaders. LOST of friction, and very hard to turn on carpet etc.
The 2-wheel versions like the one HERE: with a caster wheel have very low friction.
But in Sumo you do need traction to "Push The Other Guy" -- right??