Personally, I develop with the latest Ubuntu LTS on my machine and use either Debian stable or the latest Ubuntu LTS for servers.

If we plan to support Debian stable, it'd be a good idea to use the dot deb apt repo to get newer packages. But testing has always been stable enough to use on a laptop, so only supporting testing would provably be the easiest way to support Debian.

As for Ubuntu, we could either support all the versions currently supported by canonical, or an arbitrary pattern (like the aforementioned latest LTS and latest post-LTS).

I personally vote the latter for both.