Debian unstable (sid) - up-to-date, but unstable
Sid isn't as up-to-date as the name suggests.
My biggest issue with Debian is that it's only a cyclical rolling release, not a true one. When the feature freeze comes 'round, it affects sid as well as testing. Keep in mind that the freeze lasts for 6+ months (the current one has been 8 so far - http://wiki.debian.org/DebianWheezy), and happens once every 2 years; at least 25% of the life cycle is spent in freeze. This is equivalent to being stuck on the previous release of Ubuntu. e.g. I'm still using KDE 4.8, despite 4.10 coming out last month.
Freezing makes sense for testing, since it becomes the next stable, but there's no reason that sid/unstable should be frozen as well.