The copper one DOES perform better. No idea where anyone would get the idea that aluminium would be more thermo conductive than copper. The smaller fins on it also help with dissipation. You'll see about 5 degrees less with an older style CPU heatsink.
If you read what I wrote with ATTENTION you will notice that I was saying that the aluminum heatsink is MECHANICALLY
better than the copper one.
Because the amount of copper on the copper heatsink is so much smaller than the mass of aluminum on the aluminum heatsink they perform almost exactly the same. With a worth mention that an copper heatsink with a failed heatpipe (leaked or whatever else causes it to stop flowing the gel inside) will cause the CPU to operate on absolute maximum temperatures and fans will run at higher speed than normal when on automatic.
So for the purpose of installing as a GPU HEATSINK the aluminium heatsink is better due to mechanical performance