I have 2 refrigeration compressors. One 5 h.p. 480 volts, 3 phase @ 4.5 amps. One 5 h.p. 230 volts 3 phase @ 20 amps. Which one will cost less to run assuming electric costs are same?

This is a real tough question to answer because of a couple of variables we don't know:

power factor - should be on nameplate
% loaded
Duty cycle

On the surface a 5 HP might theoretically cost more to operate, but if they are equally loaded and not at the extremes then look at the difference in the power factors first. This would be a number between probably 0.7 to 0.9. In any event, between 0 and 1.

The 480 V unit generally would require less cabling costs. The contactor would cost less too because the current is smaller.