First off (someone correct me if im wrong here), but there is a trade off when using copper over aluminum. Copper conducts heat better than aluminum, sure, but aluminum is far better at DISSIPATING that heat once it gets in the metal. So its not that copper is automatically better, its just better under certain circumstances.
As far as the copper tarnishing, if you applied some type thermal compound between the heatsink and the cpu, then there should be no oxygen to react with in that area and you should be fine.
No, copper is better, all the way around. Conductivity and specific heat. But aluminium doesn't oxidize as quicky, due to a lower electronegativity. You can do like 'public works' does the main water lines, and hook up a magnesium block, but if you do that people will think you're strange.
Grab a copper one if you've got the money-hunney, otherwise go with alu-min-ium.
Oh, the common misconceprion. Metals do not "give-up" heat better than any other metal. True, aluminum does <b>radiate</b> heat better, but radiation is completely unimportant in cooling because it plays such an insignificant role. Forced convection is how heatsinks release heat, and material does not effect convection. Copper is better in every respect. See the thread, "The truth about Aluminum and Copper heatsinks" for in depth info.
- Tempus fugit donec vestrum relictus tripudium. Autem amor praeterea magis pretium.
Copper is better, but unfortunately many copper heat sinks due to their size and design don't cool as well as aluminum ones. I hear the new global win copper sink is bad ass though. Also the swiftech with the copper base and aluminum pins is very good as well. Don't get the hedgehog though- heard too many bad things about it.