I won't bore you with a lecture on metallurgy, but here's a good site: http://www.ez.org/aluminum.htm. I'm using 5052 H32 and definitely would avoid the 6000 and 7000 series. They are much harder alloys and are more difficult to work with. In fact, our robots use 6061 T6 for the chassis and 7075 for the track frames - and they are built to crawl the walls inside power plant boilers.
"They laughed at Columbus, they laughed at Fulton, they laughed at the Wright brothers. But they also laughed at Bozo the Clown." - Carl Sagan