Thermal, by all means! The thermal clutch basically freewheels all the time until the air coming thru the radiator is hot enough to cause the bimetal spring in the center to move a valve inside the clutch and lock it up (which is when you hear the fan roar). So the fan is not drawing power from the engine until it's needed. The non-thermal (or 'torque limited', I think it's called)just seems to mush along, never freewheeling and never fully engaged, either. They don't do a very good job of cooling a heavily loaded or tow vehicle, I know that.
