There's a lot to unpack here.
Firstly, horse power (hP) and Torque are not interchangeable. Horse power is your measurement of speed. A 1hp motor is capable of faster speeds than a 3/4hP motor. I know the listings advertise "1hP" or "3/4hP," but unless you're looking for a faster motor, then I wouldn't concern yourself with it. More importantly they list "550W" or "750W;" let's set our focus there.
Your torque is your rotational force. How much force you require will depend on what you're sewing; not how fast you sew it. A heavier leather will require more force for the needle/machine to overcome than a lighter-weight canvas. With a brushed motor, the slower the machine runs the less torque you have. The slowest speed on a brushed motor is pretty useless. For a brushless motor you have a great amount of Torque on demand from the slowest to the fastest speed.
Back to the Wattage, which has a direct relation to the Horse Power (hP) and Torque (Nm). Wattage is Power, the more Power then the more hP and/or Nm you have. The pedal that connect to these motors are actuators. They're actuating the amount of current flowing through the motor. The size of the 750W motor is large than that of the 550W motor, so it has greater torque at lower speeds. It can also handle a greater amount of max Power, so it can run faster (ie; 1hP over 3/4hP).
The price difference between the two is minimal. If the extra $20/$30 is too much, then go with the 550W motor. No matter how powerful the motor you use, your machine will be the limiting factor as to what your able to sew. If your motor has enough Torque to overcome half an inch of veg tanned leather, but you're working on a light machine, then you only risk damaging your machine.
