Sid has also done some odd things in the past couple weeks, such as removing slocate from the repositories. It took me a few minutes to figure out that I ought to switch to mlocate. The primary drawback before that was the delayed security updates – due to the Lenny release process, the Firefox (well, Iceweasel) 3.0.5 upgrade took perhaps two weeks to enter Sid, leaving me with the vulnerable 3.0.4 for the period. Ubuntu seems to offer swifter updates.
A minimal install of Ubuntu is very nearly as lightweight as Debian. It provides a stable base and a clearer upgrade path than Sid, albeit at the cost of packages that are behind the cutting edge. Security updates for any packages in Ubuntu's main repository seem more timely than for those in Debian Sid; however, a great many lesser-used packages in Ubuntu's universe repository get no security support at all. Overall, given the issues I suffered when mixing bleeding-edge packages from Debian's Experimental with a Sid-based system, I believe Ubuntu is a better fit for my needs.
Regardless of which I settle on, both are a great deal more fun than Gentoo, which I ran for five years before switching to Sid in September. The novelty of being able to upgrade all packages in five minutes instead of fifteen hours still fills me with joy.