Should I get Windows Vista or Xp? Look at details.

I have to run an engineering program called Auto-CAD on my new laptop that I'm about to get, but I don't know if I should get Vista or Xp. All I know is that I can get an Xp laptop for like 400$ "off a truck" instead of paying 1100$ so should I get that laptop and just install Windows Vista on it or should I just leave the Xp on it?