Being a watercooler, I know that the properties of those two metals differ in that copper draws and retains heat better but aluminium dissipates it quicker. Race car rads are mostly copper. So, I suppose copper is more efficient in taking heat from the coolant loop but would need more intensified airflow to lose it.
Ideally, a copper rad with an aluminium attached piece in the area of airflow works best. Well, at least in pc watercooling. Not sure if such a car rad exists though. I've been thinking about this problem, in relation to heat dissipation mods, for a while now. But you need fans or airflow to have any real impact on temps.
Here's an interesting read.
