Found this while surfing and couldn’t resist posting the link. This guy did a massive array of tests on pretty much all versions of Windows that matter at the moment. The short version? If it’s performance you’re after, there’s no clear winner, since they all have their strengths. Overall, of the currently-supported OSes 2008 server seems to have the edge, as indicated by my own experiences. Attaching the results below, but here’s a link, too.