Well naturaly i would have gone for copper because its what all the highend pieces are nmade out of, waterblocks etc.... Aluminium is whats used by the lower end like discount HSF combos etc....
*EDIT* - i just read that PDF, that quite interesting... but the fact still remains that copper is a better conductor, it just doesnt show on low scale projects, seeing as you are using a fanless heatsink, i dont think it would actually matter, seeing as the air flow will be way WAY below 400 linear feet per second