Two types of radiators that I know about: aluminum and copper/brass. I hear that one is better than the other depending upon who I talk with. Basically, I want to know which will transfer heat the best. My other concern is paint that is normally applied to the copper/brass radiators (where aluminum is normally bare) and how that affects heat transfer.

 

Well, Copper has a higher thermal conductivity (401 W/m.K) than Aluminum (237 W/m.K) and Aluminum (897 J/kg.K) has a higher heat capacity than Copper (385 J/kg.K).

I found those numbers on a quick google search and I won't pretend to know what they mean, but I do know this; copper weighs about three times as much as aluminum. Also, if you are buying a modern radiator design from someone like BeCool or Griffen, it won't really matter anyway; because the design improvements are where the "magic" is.

So... six of one, a half-dozen of the other...
My opinion...? Go with an aluminum radiator for the weight savings.

As to paint, well... I'd imagine that any barrier between the metal and the surrounding atmosphere will impede heat transfer, although I don't think it will make a huge difference. All things being equal, I'd say that the advantage goes to aluminum again.