You will get a lot of different answers to that question.
The reason being is that no two Windows systems are a like.
However, at the base level of installations, Windows XP will be faster than Vista. Overtime, they both become pretty ugly, but Windows XP has a better chance of becoming uglier as Microsoft has tightened up the default security levels in Vista versus Windows XP.
Anyone can mimick those settings in XP if they wanted to.
You did not ask about stability, and useability, so I will leave it at that.