The heuristic I use when choosing between them is: What is the server supporting?

If it's legacy PHP apps (like SquirrelMail, or SugarCRM) that need specific versions of PHP and mysql to be happy; then debian.

If it's more modern apps, or things that are currently in development Ubuntu. Ubuntu's python and postgresql support are excellent.

Interesting, that seems exactly what my situation is. Legacy apps are on Debian and considering moving to Ubuntu for new app using Python / Postgresql.

I use Debian for most of my servers and the only complaint I really have is that the repos are extremely outdated when compared to Ubuntu. Ubuntu's main strength, at least as I see it, is the fact that the packages are updated more frequently and there are more of them than for on Debian (even Backports).

Yeah I can always count on Debian to be dated but for the most part I have been able to work with it despite that.

Also, Ubuntu has LTS (long term support) versions. The server versions are supported even longer.

I favour ubuntu on servers mainly because it's what I use on my desktop, and it's handy to be able to write & test scripts on your desktop and deploy to the servers only once everything works.