Hurst motors are very inconsistent; some are quiet, some vibrate more, some start off quiet then get noisy.
If the motors are the same power rating, there should be no difference between 300 RPM and 600 RPM motors. VPI used 10W 600 RPM motors and 5.5W 600 RPM motors. AFAIK, all the 300 RPM motors are 7.5W so it will be noisier than the 5.5W 600 RPM, but less noisy than the 10W 600 RPM.