The limiting factor in removing heat from a heat sink into the air is the surface area of your sink and the thermal capacity of the air. The thermal capacity of copper is greater than aluminum so heat sinks of equal mass, copper wins every time. But, since copper is denser than aluminum equal masses mean there will be more aluminum by volume. If you make both to have the same surface area, copper still wins out. That is because it has a greater thermal capacity than aluminum. Heat moving into or out of a mass is the same reaction, just in reverse.