Watercooling Myths Exposed
 

Myth: Aluminum absorbs/dissipates heat faster than copper.

Reality: All thermal properties of copper are better than aluminum. Aluminum's advantage is that it is lighter and easier to machine. So, if one were given a pound of copper and a pound of aluminum, you might make a better performing heatsink with aluminum, as it might have more surface area to dissipate heat than copper for a given weight. Considering footprint limitations for air cooled CPU heatsinks, however, copper is definitely favored.

http://www.overclockers.com/articles1088/