Sabertooth 2x12 PWM vs Serial

Hi, Can anyone familiar with the Sabertooth 2x12 motor controller tell me the differences are between the various modes, in particular serial vs PWM? What are the advantages of one over the other?

I have an RC truck that i am using as the chasis for the my robot. I stripped everything but the motors and have connected those tot he sabertooth. I am currently using PWM and all works fine except when turning. my robot performs tank style turning and the problem here is that it doesn't seem like their is enough power to turn the left/right wheels independently. I am using a 9.6v 2200ma battery...which i think should be sufficient to power the motor (i know that before stripping the motors, the truck had no problems spinning in place). So my thinking here is that perhaps using PWM is just not delivering enough power in this case.

please advise.

Thanks much

Are you using an R/C filter and faster PWM speed as suggested by the documentation?

It would probably be easier to use it in RC mode (servo commands) with the Servo library to avoid those above issues.