The 350w can handle less power than the 500w one. Choose it if you want to run 22 amps or less. It's the same size and weight as the 500w one, so get the 500w one if you want to go over 22 amps. Hardly anybody sells BPMs by code numbers. Instead, they normally give rpm, though you can never be sure that the rpm is correct. When you find one with the correct rpm, you should ask the vendor for the code number to be sure.