I'm dual booting XP Pro (SP3) and Vista Ultimate 32 bit (SP1).
XP hands down. I rarely use Vista. I use Fangui in both OS and the fans never stop working in Vista, but rarely in XP. My 1710 runs so much smoother in XP as well.
I actually bought the XPS with Vista pre-installed and then decided to give XP a go about 2-3 months ago and never looked back. I'll keep Vista there, "just because", but I don't see myself using it any time soon.