Both- sort of on a hub motor
Watts is like HP on a car. Ultimately it determines your top aero limited speed
Volts works more or less like gearing to a hub motor. Winding out at too low of a speed, but have plenty of power? You need more volts. (Note no mechanical advantage of real gears.
Amps work more or less like torque on a car. The more amps, the quicker off the line you are.
Just like a car can make HP through RPM or Torque, a electric motor makes watts through volts or amps.