I only have those two choices because the laptop is purchased from my law school, and I need it immediately.

Functionally, XP Pro SP3 is supposed to run better than Vista SP1. However, since XP is pretty much an outdated system, I don't think they will have any more updates, so I don't know if it will be good if I'm going to use it for at least 3 years. Then again, is Vista getting replaced soon with uh, Vienna as well? I don't know. Vista SP1 that they're offering is 32-bit as well, and there is no price difference with what system I choose.