Things that really can make a difference in performance is the design of a heatsink, pins and fins all try to make the contact surface with air as big as possible and how bigger the surface how more it can spread his heat and copper does this better. Another very important thing is the fan, a powerfull fan will draw the heat away much faster than a weak fan. Also a good contact between the CPU and heatsink is important! Lapping or thermal compounds are used to make the surface as flat as possible. So between Copper , Aluminium with Copper base and Aluminium heatsinks with exactly the same design and fan the Copper one will be the best, and the Aluminum one the worst. But the difference will only be a few degrees.