What's more important for building a vibratory polisher, the frequency of the vibration or the amplitude of the vibration?

The reason I'm asking is that I need to decide what size pulley's I need to machine for a small polisher I'm building. The motor I'm going to be using is a 5/8 HP 30k rpm motor, and it will be vibrating a 5 gallon bucket. Should I gear the motor down for more torque and larger displacements, or keep the ratio close to 1:1 for high frequency?

This polisher is going to be used for mostly polishing aluminum, and maybe deburing the occasional piece of steel.

Also, has anyone ever tried garnet abrassive for polishing/deburing?