for you information I've been looking into the properties of metals for quite some time now and have looked at a extremely large number of web sites on this. As for conduction your right it is important for a heatsink to have both conductive and radiant properties this is why we have copper embedded heatsinks. This allows for heat to be conducted to the Ai which then allows better dissipation.
I have have seen several reliable web sites (mostly ones run by people with a degree in this area of science)that have provided the thermal resistance of both Ai and Cu. All of these sites have show Cu to have a higher thermal conductivity than Ai but also have stated that Ai has more surface area and therefore has more surface area.
I a heatsink one of the critical factors is surface area. If a heatsink has a large surface area this will allow for better cooling simply by the natural flow of air (heat rise etc.)
this can explain the variations in results because if one person tests a copper heatsink in a case with many blow holes the results will vary from that of someone who tests that same heatsink in a case with no blow holes.
As far as a metals ability to radiate heat Ai will always have more surface area than Cu. The only way to increase the surface area of Cu (to the best of my Knowledge) would be to make some sort of microscopic grooves on all surfaces of a heatsink. Since this would be so expensive I believe the heatsink that would preform the best at a reasonable cost would be a Solid Copper heatsink with a vary thin layer of Ai over the solid copper heatsink. This would allow for the for minimum thermal resistance and maximum surface area.
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