I'll give you some input.
I've been running the X12 17.5 & tried the 13.5 a few times.
if you are not the best at gearing the X12's will be more forgiving they tend to handle heat a little better.
Lot's of pop out of corners and are pretty smooth.
the DOU 3 seems to have a lot of RPMs. I'm not sure about the heat handling, go to the Duo 3 thread for input.
I tried the motor and it feels great plus it looks freakin' nice.
at the end of the day with a Tekin RS you can't go wrong either way.