Well I think it depends on how you work with Windows. Bootcamp is of course free, but you have boot into it at startup and then your computer becomes a Windows computer which means it's prone to all of the Windows faults and enemies and so on. I would use VmWare if I were you for the virtual environment it provides, but it's $80. I don't use Windows at all anymore (except to troubleshoot my wifes pc), that is why I got a Mac.
I would stick with XP for this application, but again that's a personal preference. I have used Vista and hate it.