Actually, Copper is supped to provide better cooling than Aluminum. That is if the whole cooling surface of the aftermarket radiator was built out of cooper. Which I am pretty sure is not. Last time I checked, the fins looked like they were made of steel. Please correct me, if I am wrong.
Here are the thermal conductivity coefficients for some materials (W/m*K)
Silver 422
Copper 402
Gold 298
Aluminum 226
Steel 73.3
Lead 34.8
The best choice for a radiator material would be Silver.
Off course design of a particular radiator matters a lot too.