It depends.

For different types of robots we have two different model wheels. At this article I will handle differences  of rubber and silicone wheel models. first of all silicone has better traction than rubber and nearly for all types of robots we recommend our silicone wheels. When they rubbed with alcohol they have superior traction against wood or plastic surfaces. 

Our hard rubber models have only one usage area. They are for magnetic type sumo robots. Magnetic Robots are only 3kg at hand. But when they are on metal dohyo robots can hold surface with 50-100 kg force. This force will change silicone wheel's structure due to softness (Shore A 20) material. Where as rubber wheels are hard like 60 Shore A type hardness. At this hardness wheel's outer structure remains same diameter.

One advantage of rubber is high resistance to wear.

So in short, use hard rubber models for magnetic sumo robot, and silicone wheels for non-magnetic sumo robot.