Regardless of 50 or 60 amp, same wire is for both- use 6 gauge copper.
Only difference is what size breaker to purchase. You could go with a 60 amper, and if that's how your spa is setup, you'll know if all of your pumps work at the same time, and still allow your heater to come on.
If this doesn't happen, you are probably set for 50 amps, and could easily get the jumper configured correctly to allow full 60 amp operation.