Copper is a better conductor of heat, and more flexible than aluminum. It is also much more expensive. In the old days, most, if not all, radiators were made with copper cores and steel or copper tanks. They did not react to impurities as badly as aluminum. The tanks and cores were soldered together with lead solder and could be easily cleaned out by removing the tanks and rodding out the cores. The aluminum radiators use plastic tanks that are crimped on and are not as good.
