I have a little problem. I am currently running Windows Vista Ultimate 64 bit. I also have XP 64 bit sitting on my desk. Which one should I use?

Current specs:

Intel E8400 @ 3.0ghz
8GB RAM
Lots of disk space
Samsung T240
Creative X-Fi Platinum
ATI RAdeon H4870 1GB

Vista seems to use a lot of memory at the moment so not sure if I will get better performance with XP?