I would rent on STJ unless they expect use the car on STT during the trip. I would also suggest 4wd not because you need it for off road but the switch backs can get slippery in the rain. We have used Varlack and Conrad Sutton. Both were fine. I would ask whether the rental company will provide free parking in Cruz Bay when in town. Take photos of the vehicle when checking out. Don't take the top off and don't leave excessive sand or you will pay a surcharge on return.