Debian Testing or Debian Sid?
There is an interesting part of Debian GNU/Linux FAQ devoted to this question. In particular the choice depends on
- security/stability considerations
- expertise of the user
- need for newer versions of software
- support for new hardware
I would like to point out the following passage from that page:
Stable is rock solid. It does not break.
Testing breaks less often than Unstable. But when it breaks, it takes a long time for things to get rectified. Sometimes this could be days and it could be months at times.
Unstable changes a lot, and it can break at any point. However, fixes get rectified in many occasions in a couple of days and it always has the latest releases of software packaged for Debian.
Using unstable implies you "know what you are doing". You have to be able to fix problems if they occur. Unstable tends to break occasionally in a major way. Not everyone has this level of expertise. In general, I advise people to run testing, which practically never has the major problems that unstable has, since problems with packages typically get caught on their way through unstable. I think this is a case of, if you have to ask, you shouldn't do it. :-) Also, using testing is better towards the end of the release cycle, once it is frozen and on route to becoming the next stable.
Having said that, why not run stable? I do. If you want more up to date package, you can backport to stable to a limited extent. There are some packages available at the official backports site, or you can do it yourself.
Packages arrive into testing after they have been sufficiently tested on sid, specifically, two weeks without any outstanding bug reports. Testing is an RC version, so to speak. It is typically a few days behind sid in terms of the latest software.
If you're not doing any Debian-specific development on your machine, go with testing. If you're planning to contribute to Debian, you'd be better off with sid, since all new changes have to work on a sid environment.
That said, I have been using sid on my desktop for over a year without any crashes.
Details: http://www.debian.org/doc/FAQ/ch-ftparchives#s-testing