What do you mean by "a beater"? If you want a bike that you can ride around town, use as a commuter and lock up outside your favorite bar or library, then alum might not be the best choice. Alum frames tend to have very thin walls, and dent pretty easily. They are great for road riding, but you have to be a bit careful that you don't drop them, or that someone doesn't drop something onto them.

If you still want an alum frame I wouldn't worry too much about the type of alum and instead consider the manufacturer. Cannondale is a favorite around here for alum with the CAAD series, but most manufacturers make alum frames. There are also a lot of cheap alum frames (Bikesdirect or performance for example) that are very ridable and could be easily replaced if crashed or dented.