Copper is actually a better thermal conductor than aluminum. The problem is that the copper heat sink is not all copper. The tubes and fins are copper, but they are bonded together with lead and have a terrible heat transfer capacity. The end tank is made of brass and the side grooves are made of steel. Better cooling techniques are wider tubes and shorter fins. This added"tube to fin" contact area determines the efficiency of the heat sink.
A typical 4 row copper radiator uses 7/16" tubes while the aluminum radiators use two rows 1" tubes.
The copper heat sink then uses 1/2 high fins, while the aluminum heat sink uses 3/8" fins. Shorter fins increase the number of layers in a given stack. For example, a 18.5" high heat sink has 30 layers are used for copper units relative to 40 layers of aluminum. Therefore, each layer stacks more layers and each layer has more surface area.
The aluminum heat sink is 100% aluminum brazed and does not require any insulating solder. This is why aluminum heat sinks are better than copper heat sinks. If you can make a copper heat sink like an aluminum heat sink, it will work better than aluminum. It also weighs about 90 pounds.