The 300rpm motor is quieter.
Try this: take the belt off the motor so that the platter won't move, lower the stylus onto the platter, switch on you amp and turn up the volume. Now switch on the motor and you should hear hum.
The hum will come and go as you switch on and off the motor. The hum isn't electrical, it is from the vibration of the motor transmitted through the table and shooting back up to the platter from the cone feet.
If you try this experiment with both motors, you will find that the hum from the 300rpm is much much lower.