I have Vista business on my R&D machine and my development machine has XP pro. I hated Vista initially - but I must confess it is very good - once you configure it according to your needs. I have switched off all irritating messages and am able to run applications in backward compatibility mode now. The speed factor and feel good factor is more in Vista than XP pro SP2.
Also, be very sure that Vista is the Microsoft's roadmap for future OS versions and XP would be discontinued in early 2008.
So I would bet for Vista as now a days almost all new hardware devices have Vista support drivers. Of Course I have an excellent graphics card and 2 GB RAM to support Vista
