6in or 8in?

[Zholl]

It's ultimately going to come down to what you're wanting your drive system to do. 6" and 8" wheels are going to require slightly different gearing and are going to have somewhat different torque and speed outputs. In addition, the plaction wheels are most likely going to have a higher coefficient of friction than the KoP wheels, though I haven't tested this so don't quote me for truth on that. I would certainly believe that the softer rubber on a plaction is going to have more grip than the harder rubber of the kit wheels, though.

If you don't mind doing a bit of reading, I would recommend this thread discussing the differences of different wheel diameters. They go a bit more in depth than I have here