The main advantage with 8 wheel drive is little to no rocking, since the robot rests parallel to the ground on its middle four wheels. It also helps getting over large obstacles (read: 2010).

Small wheels are amazing. You don't need nearly as much reduction to get good speeds, they're light, allow a lower center of gravity, and are generally cheaper than large wheels. In fact, I've yet to hear a solid argument as to why any wheel size above 6" diameter is needed. 4" works great, but 5"-6" wheels are sometimes preferred to get over game pieces.