Which last longer underground?

Wendy,

If you are talking about uninsulated electrical cables, then the first thing to know is that bare Aluminium wires are not listed to come into contact with the earth, because they oxidize.

However, Insulated Aluminium wires, cables are allowed to be buried.

That's what most Power Companies use for the undeground cables that run to the home from the transformers.

By the way, the NEC requries that all current carrying conductors be insulated.