[lincpimp]

So you are running buggy ratio diffs? And what size spur?

If you are running what I think you are I would go with the 1520 and 6s. More torque so you can load the motor more with a larger pinion. Or go with the 1717 and the biggest pinion you can find. I like the 1717 motor, and Pdel seemed to think it was between the 1521 and 1527 in power output, so it is more powerful than the other options you listed. Speed needs power.