My almost 20 years old experience is that it's the worst of all Debian worlds, and none of them are good in any case.

You would be neither getting distro's promised stability, nor would you be close enough to getting upstream's fixes like sid/unstable.

I vaguely remember Gnome's bug tracker filled with duplicate reports about crashes that are long fixed upstream, but also don't trigger in either stable or sid Debian packages. They all came from Debian Testing. Gnome must have been very happy with Debian enabling the crash reporter in their packaging!

Do note that sid/unstable itself becomes as shit as Testing whenever the next Debian release is around the corner, as the project freezes the world in that period (or used to at least).

If you want the imagined stability you guessed you would get from Testing, you can use Ubuntu. At least with that you won't get packages that are left crashing for everyone. Or honestly, if you're on a desktop, just use Arch. The supposed meme about using Arch exists for a reason.