First, MC cables can be used outdoors or underground regardless of whether they are copper or aluminum. Both cables have similar constructions, with the only difference being that they are made of copper or aluminum strands.
Aluminum MC cable is made from THHN wires that have been covered with aluminum metal clad armor. The MC cable can be used outdoors or in conduit because all the wires have THHN insulation. Romex cable also comes with THHN wires, but the ground wire isn't insulated so it can't be used outside.
To keep it dry, the aluminum MC cable can be outfitted with a PVC jacket. It can be used underground without the need for a conduit. Good quality 14 gauge solid copper wire versions are also available for outdoor and underground.
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This is the cable's make-up. But why would anyone choose aluminum over copper?
Most electrical inspectors use copper as a conductor of electricity. Aluminum is cheaper and lighter because of the price of each commodity, which then reflects pricing for the cables. You can use aluminum in your application if you have a gauge that is at least two times larger than the copper gauge. Your electrician will explain the conductivity difference to you.
The aluminum version is a better option, even though it has a larger gauge. This will result in a lower price for your cable. Due to the difference in weight between copper and aluminum cables, shipping costs will be lower.