I've used Debian unstable and agree, but given that most packages in testing usually only lag behind those in unstable by a few weeks... any particular reason?
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.
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...
