Doc; Jim's right on with his reply. I've used both copper and aluminum for new services and updating old ones, including adding subpanels. The Al is cheaper than copper, but a little harder to work with. For a service or subpanel, you have to go one wire size larger with Al than copper. I do beleive that has to do with the Al expansion/contraction issue, that overtime can loosen up the connections. I prefer copper, you can twist it up pretty tight, since it's not as brittle as Al.
As of this time, I believe also that Al is not approved for branch circuit use. I don't think your subpanel would fall under that restriction, so, I guess for your use, it would depend on how much you would save with Al, opposed to the little extra work you'd have to do. I've no problems running Al, but like I said, I prefer copper.