Sorry Rosie this doesn't answer your question, but we found a great EASY product to wax your car. Try the brand Turtlewax car wax wipes. They are in a plastic cyclinder in the auto department. Found ours at Kmart. They are quick, and when we did get rain the water beaded up just like a regular wax job would. (You have to apply to a clean car though.)
Both compounds contain grit, but in rubbing compound the grit is more coarse. If you ever have a vandal spray paint your car, rubbing compound will remove his artwork. Call your local bodyshop, they can tell you when to use which one.