A personal view of course, but it really is no contest. Vista outscores XP in every department, and why even consider an operating system that is now obsolete.
Also I think it will be more than two years before any replacement comes along.
Worth checking out is any existing hardware that you want to continue using - printers etc. Check the manufacturers web site for Vista drivers. As for existing software all you can do is try it. I had just one program that wouldn't install. A small price.