I'd say go for vista.. IIS7 is great, and the memory allocation is smarter (Vista doesn't waste RAM, it fills it with what it thinks you will use in the near future). As for the background services... you can turn them off like you could in XP.
However... if the rest of the company is on XP, I strongly suggest you install an XP virtual machine so that you can test your apps on the same platform as everyone else.
