Choosing between Debian and Ubuntu
- Do you want/need 'conservative' stability -> DEBIAN.
- Do you need latest software and are you willing to run maybe into some instability issue -> UBUNTU SERVER.
It is a matter of taste. Personally I would always recommend Debian as a Server OS anyday and the latest -> Debian Squeeze has most of the recent packages you would need since it has been released pretty recent.
I use it myself as a XEN virtualisation mothership running up to 6 Machines on it with a second backup machine synced via drbd. I am more than happy with it.
I'd say Ubuntu Server LTS is a pretty good compromise between the stability of Debian and the 'latest and greatest' of the latest Ubuntu.
Ubuntu is effectively Debian testing with some specials on top and some bugfixes from Debian testing backported into it. Since the current release of Debian is quite new, you get fairly recent release of most packages. And there are also backports for the more important fixes. For a heavy duty server I would always go with Debian, especially if you already have experience with it.