The Professional KitchenAid mixers (i.e. the ones with bowl-holding arms) are all metal gears, with an electric controller to protect the motor from overtorque. The tilt-head models are mostly metal gears, but have a single kevlar gear that's designed to break to protect the motor from overtorque.

Stripping the plastic gear in the mechanics is like blowing a fuse in the electronics. It's possible it was a bad component, but it probably means you've overloaded the circuit. If you need more torque, buy the models with the bigger motors designed to handle it. The main trade-off between the various mixer models is basically price vs max torque.