I'd recommend brass over aluminum. Brass can be annealed easily by heating it to red heat and quenching in cold water; it can be soldered very readily with silver-bearing solders that are nearly as strong as the metal, cuts and drills nicely with common metal working equipment and doesn't clog grindstones and files as badly as aluminum, and IMO looks nicer than aluminum (which to my eye gets gray and ugly as it ages). Brass repolishes better and isn't prone to galling as most aluminum alloys are. Brass comes in a number of useful rolled and extruded shapes (round, square, and hex tubes, angles and channels, as well as strips and sheets), available in small pieces at somewhat reasonable prices -- many more such than aluminum, from which extrusions are typically available only in shapes and lengths suitable for making screen doors and such.

It is important to anneal brass before trying to bend it; most stock purchased off the shelf will be work hardened from being cold rolled and thus prone to split when worked further. Just heat the stuff up to dull red heat (don't go much higher, though, or keep it hot too long; the metal melts just above red heat and you'll slowly drive off zinc from the brass even at a dull red), then drop it in cold water. To reharden later, if needed, heat the brass red hot and allow it to air cool.