Holmes, actually industry standard PTZ cameras use stepper motors driving a belt attached to the cameras gimbal. Since the OP mentioned that the camera is heavy weight this would still be the correct approach to take. though if the manufacture of the housing and gimbal mechanism is going to be problematic for you (im lucky enough to have a metal workshop at home), then two servos may be the way forward for you particularly if you're new to all this. It would make the project a lot simpler for you.