If your only choice is between XP (SP3, I assume) and OS X 10.6.x and your goal is to keep the total cost of ownership as low as possible, then I think the decision is a rather obvious one: OS X.
If however it's an option to upgrade the entire network to Windows 7 and thus have a uniform environment, then you might want to research into this a little further.
As for software, I can't really comment, but with the right tools, Macs are still more versatile than pure Windows machines, even if it means having a dual boot system with OS X, Windows.