thermal or not to thermal

hi, i'm rebuilding a 360 to put into my 70 duster. i came across 2 different fan clutches, a thermal clutch and a non thermal, what does a thermal fan clutch do that a non thermal one won't? which one is best for a performance app. thanks :salut:

the non thermal is known as a torque drive. it loosens up as the rpms increase, and tightens up as they decrease. the thermal works with air temp, loose while cold, tight when hot. the MP viscous fan package uses the thermal style. I run that and it works just fine.

Non - thermal is the performance one. It all depends what you are going to do with the car.