[Butcher]

Given the same design Cu should be better regardless of airflow (unless it's low enough that radiation is a major factor than black anodized Al is better). However because Al is easier to machine it's a lot easier to get a better design with Al which makes up for the reduced conduction (eg. a lot of Cu hsfs are press fit or soldered whereas Al hsfs tend to be extruded in one piece).

Also for the 5000W stuff mentioned, it's probably not such a small heat source as a cpu is proportional to heat produced which means heat spreading is less of an issue. Also the weight issue is going to be a major factor in something that large which makes Al a much better choice.