What PWM frequency would give higher torque or RPM for a Brushless DC Motor
PWM frequency has no effect on maximum torque and rpm, because at full power the PWM ratio is 100% (full on) so there is no frequency. At lower PWM ratios the frequency has an effect on motor efficiency and throttle linearity - the lower the frequency the worse it gets.

PWM controls motor speed by switching the power on and off rapidly with a varying ratio, so the motor is getting getting a series of full power pulses. At low PWM frequency this causes extra loss due to vibration and current surges, but the increased rms current and voltage at mid throttle causes the motor to produce more torque and rpm than it would with the equivalent DC voltage. So you might think that it's more powerful, but that's only because the throttle response is non-linear. In reality more power is lost due to the higher rms current heating up the windings more, and the vibration wasting energy in the gears etc.   

As PWM frequency is increased the motor's winding inductance begins to smooth out current flow, so the motor runs more efficiently due to lower rms current and torque ripple. On the down side, higher PWM frequency causes higher loss in the controller. So you should choose a frequency high enough to make the motor run smoothly without making the controller get too hot. Inductance is proportional to number of turns, so the 13.5T motor could get away with a lower PWM frequency than the 4.5T. However if the controller can handle it I would just choose 16KHz for both.