I agree, and what's more, copper 'picks up' heat faster than alu, but it needs more airflow to get rid of it. Alu needs very little airflow to dissipate its heat into the air, so if you're using a low rpm-fan (like with this heatsink), you're better of with alu than copper.
Oh, no don't bring this kind of old myths back to life.
There is nothing magical about heat transfer.
Copper does transfer heat better than aluminium per area. That includes both "picking up" heat and giving away heat. So the same size and shape heatsink made of copper should always do better than the similar heatsink made of aluminium. Tests with the Zalman 7000 heatsinks show no difference in forformance between the Cu and AlCu versions, sometimes even in favour to the AlCu. This does not just have to do with the material, there are lots of other factors. Like for instance that the efficiency of fans can vary 10% between samples.
From reviews we can clearly tell that the cost difference between the Cu and AlCu version of the 7000 does not justify buying the Cu version.
To be objective I should point out the pros with Aluminium as well:
It weights a lot less than copper.
Per weight it transferes more heat than copper.
It is a lot cheaper than copper.
It is easier to work with in manufacturing processes, so the total cost of finished products are less.
In general it is bad to produce with mix of metals since you get more material joins, but in reallity we see very good performance compared to weight and cost on all AlCu heatsinks. In fact I use AlCu heatsinks on most of my computers and are very satisfied with the performance.