well actually looking at it from a buyers stand point, what are the stats of the servers the VPS are hosted on and how many Guest OSes are on each server.
also after playing with win2k3 compared to a linux server distro(ubuntu), windows might not be as bad as i initially thought when i first read the post.
the GUI of windows does take more resources, but most server applications shouldn't need to worry about how much resources windows is taking up, compared to what is avalable. if windows is affecting your performance, then it might be a good idea to upgrage.