I personally would go with Windows 7, but you may encounter a bug or two since it's still beta. Of course you're guaranteed to hit plenty of bugs with XP since it's so outdated. I've been running Win7 since the first beta and have had no major problems with it. I've also never had any major problems with Vista Ultimate either. I actually really like Vista, though it is bloated.
Vista's resources consumption is abnormal, it's impossible to work with an operating system that makes a powerful laptop become "slow".
Found an idea: 2 partitions, P1 with 248 gigs for 7 and 2 gigs for XP.
Windows 7 has XP mode virtualization built in as well. The dual partitions is definitely a good idea though. I would set it up as Windows 7 and Linux, mainly because Linux partitions are so easy to access other partitions if there are any problems. I've saved quite a few beta disasters with a trusty Ubuntu or Fedora partition.