Copper or Aluminum Heat sink

Thermal conductivity of copper is almost 60% higher than that of aluminium. This means that a copper heat sink will be quite more effective at removing 

heat than an aluminium one.

Which you choose is a matter of compromise: alumimium heat sinks are cheaper and lighter, and so are the first choice for general purpose design. Howewer,

where you must remove large quantities of heat in little space, copper might be preferable.

However, it is not possible to make an absolute comparison between the two materials without knowing the specific application and the other constraints of 

the specific design the heat sink must be adapted to.

There are other factors to keep in mind (including the environment where the heat sink must "live" in).

Copper is able to conduct heat better than aluminium, but thermal coupling between heat source and heat sink, and also between heat sink and "outer world" 

should be considered.

Copper has about between 50% and double the thermal conductivity of aluminum depending on alloy, so for a given performance a copper heatsink can be 'half' 

the size of an aluminum one.

However, copper is much more expensive than aluminum, and somewhat more difficult to fabricate, so it is more costly to produce. In some cases, the small size 

is worth paying for.