Good points by everybody here. Some of the exceptions where copper would be better is running to equipment that has lugs that can only accept copper conductors, having to go up to the next size raceway where the raceway is already installed or would be a giant pitb to upsize, where wire ampacities cross on the tables, and so on.
Exceptions notwithstanding, generally you will come out way ahead with aluminum. Specifically, there are a few cases where copper is the only way to go, job spec being the number one reason.
branch circuit wiring is almost exclusively run in copper, however that is why I mentioned in my first post regarding the wire sizes. You are almost never going to see number 12 in aluminum, 350 MCM is a different matter.