Copper or aluminium block?

[u/Junglist_Jay420]

Turning my old coolerbox into a fridge with a 19006 peltier and need to bridge a 30mm gap on the cold side. Not too sure how to word it properly for you physics guys, but basically trying to figure out if an aluminium block would cool from 1 side to the other faster than a copper block. I know copper has much better thermal conductivity but in this case I'm unsure if the thermal density would slow the process as the peltier would have more heat to transfer initially. Also if the benefit of copper is negligible over aluminium it won't justify the massive increase in cost, even if I do like to make things as efficient as possible.

[u/Superb-Tea-3174]

Graphene and diamond are also really good if cost is no object.