The Pro 4 HD has more power. It's longer and matches the length of the Sc4x so it will fit all of the current SC trucks on the market without issue or modification to the truck.
The torque is next level providing the grunt some of the heavier trucks really benefit from. It runs cooler in most cases and simply needs to be geared up a tooth or two when replacing your Pro 4 motor.
Roar already doesn't allow for a 4 pole to be any longer than 540 length so I doubt this motor will be submitted at this time.