[RC-Monster Mike]

The efficiency difference is actually pretty minimal between these two motors. I think that if they are geared for the same overall speed, runtime would be about the same with either motor. The 8L would accelerate a little quicker than the 7xl, but the 7xl would run a bit cooler. Both will work quite well with 12 cells. The 7xl will be a much better option for 14 cell use, and would allow more speed and runtime with the extra batteries. For 12 cells, the 8L is a pretty nice motor, but the 7xl would get my vote due to its ability to dissipate the heat better, which will allow back to back battery packs with less heat. Both motors will be pretty impressive on 12 cells with good batteries, though. And I would use a 5mm shaft regardless of the motor used. The 1/8" shaft will need care when mounting the pinion(keep it close to the motor) to prevent damage to the shaft.