Biggest difference and the reason they cost more is because Danny had U.S. wire shipped over to have used in the SS-1 motors.

I personally have the 13.5 in my Stadium truck and SC truck, they are awesome, I run the 8.5 in my mod SC truck and just got the 6.5 for my 22-4.
I think the 8.5 has all the power I need for the track I run on and it never gets hot. he 13.5's I actually turned up the timing higher than I had my Tekin Gen2 motors and have less heat and more performance.