Once you go Debian, you don't go back

Ubuntu is a great distro, and they do put quite a bit of polish into it. But in all honesty, it's basically Debian's Sid (unstable) branch.

Nothing wrong with that, but after awhile, the whole "whoa, new feature" thing gets a bit old and the idea of staying about a year behind so the majority of the bugs can get ironed out kicks in. Bleeding edge is usually not worth it.

Summary:
Ubuntu/Debian Sid: damn whipper snappers. Firm release dates releases often contain critical bugs. releases are more like RC1 releases which they attempt to fix after it is pushed out the door.
Debian Testing: packages that have 1-2 years worth of testing that have (for me) a perfect balance of features, and stability. Very few major bugs.
Debian Stable: 2-3 year old packages, about as close to bug free and stability as you can possibly get. It's ready when it's ready, even if the target release date is missed by years.

conclusion:
choice is good.