OCIA.net has posted a 5 Heatsink Shootout. Below is a direct quote from the shootout:

"It seems that a consistent theme is being used in many of today's better heatsinks. Copper absorbs heat better than aluminum while aluminum sheds heat. Aluminum fins therefore shed the heat faster while the copper base is more efficient pulling the heat from the CPU due to its better thermal absorption properties."

Direct Link: ocia.net