I have always read that copper does a better job at transferring the heat. Here is a little bit that I found.
Aluminum vs. Copper-Brass
Many new performance radiators, such as those from Griffin and Be Cool, are made from aluminum that offers weight savings over copper-brass. While aluminum does not dissipate heat as well as copper-brass, modern radiator design more than makes up for the difference. Old copper-brass radiators were soldered together, but solder is a poor thermal conductor and this reduced the ability of the fins to remove heat from the tubes.