the 4.5 motor is very strong-i run mine with carts and it runs very smooth-to me these machines seem to work better with carts. Most rotary dont even have a 4.5 watt motor and when they do are made for carts. Infact the stock 4.5 runs tighter tension hawk carts the best.It seems to cut down on the noise and vibrations and really lets you open the motor up. I can only imagine that the 10 watt would only be use full for large standard mags like 23 and up and hawk carts with large groups.