45 has used a urethane compound for our rear wheels the past two seasons. In 2004 we used something like shopping cart wheels and this year we used "doubled up" Andy Mark omni wheels. It greatly depends on the game. We have found our smaller wheels (typically the past two years to be 4-6 inches) to help keep our CG down and power up!
It all depends also, on how you want to play the game. For instance in '04 we did not climb the stairs to get on the center platform, that would be nearly impossible with our wheel set up.
-Kyle