The JR servo to have is the 3550. Its a mini servo, about .13 s @ 4.8 V, and enough torque to turn the wheels at speed, and hold them. Its plastic geared, but then again, so are most mini servos. The only 1/12 servo Ive ever seen with metal gears is a Hitec

I dont think MG is really necessary, either. Its just extra weight.