SuperUser has an interesting question and answer page.
Ubuntu and Debian are my two favourite distos. Ubuntu has the more up-to-date packages, but Debian is more stable. I'd been toying with the idea of trying Debian Testing, to get more up-to-date packages, but the Q&A page pretty much coalesced my ideas- for me, the stability is becoming more appealing than the desire for the latest packages. If I really want the excitement of getting the latest packages reasonably promptly, Ubuntu, the way I came, would be the way to go (or in fact, return).
Ubuntu is a reasonable compromise between features and stability- LTS releases a good compromise- and Debian is an excellent choice for a rock-solid system, although the packages become a little dated as the release ages.
Debian Testing is for people who enjoy regular updates, regular new features and fixing problems. One of the things about Debian Stable (Lenny) that is growing on me is the complete lack of updates I've seen- consistency from month to month seems to be comfy rather than boring. Quite a contrast to Ubuntu, where something seems to get updated every few days.
I have to say I am looking forward to the release of Debian Squeeze, and some updated packages, possibly in the autumn (the release date is uncertain)?