[chungy]

That's kind of the same question I would have about choosing testing over sid.

If the intention is to be on rolling-release, you might as well just use sid. If you'd rather just be on rolling-release until the stable release is carved out (new features desired, for example), testing is pretty fine since it will transition into being the stable release.

[lamby]

In my experience, testing is (across the entire release cycle) often more buggy than sid, especially as it will get scant detailed attention in, say, the first year after a stable release. Using testing (for me) is usually just another annoying speedbump too - for example, one's issue is usually fixed in unstable and you are "just" waiting for it to migrate but it won't due to some boring, unrelated reason about 7-dependencies deep that might take a week or so to be resolved...