[BillP]

What you really need to consider is what you want your robot to do. If you want a strong robot that can push other robots around, I would recommend 6 wheels and as many motors as you can put together. However, if you want a quick, agile robot that will run circles around the "bully-bots", go with 4 wheels and two motors. In either case, you should look into the possibility of (at least) a two speed gear box so you can get the benefits of both strength and speed.

Good luck!