Copper or aluminium? Which one to use and when?

There are copper busbars, aluminium busbars... and copper busbars made of aluminium

Apart from their electrical conductivity, the other technologically important properties of copper and aluminium differ so significantly (density is an obvious example) that their areas of application are and have always been clearly distinct. And not a lot has changed or is likely to change in that respect. The only really novel development in recent years has been the introduction of cast copper rotor cages.
There are really only three, now four, areas in electrical engineering in which aluminium and copper are competing in the same market segments:

                                                   

Practical uses of copper and aluminium in the electrical engineering sector - areas in which both metals can be used are rare.