Depends on the type of dimmer if it will actually save you any electricity, some are just variable resistors, so they add more resistance, making the light dim, but you still pull the same amount electricity. Thermostat is the way to go. plus you get the temps you want.

You really can't get more efficient than a true rhp, majority of the heat is produced on the surface of the objects it points at, it doesn't just heat up. Flexwatt is great for racks tho and also still pretty efficient.