I personally recommended Debian 6 for your operating system

Debian 7 'Wheezy' was released this weekend just gone, and is now the new 'stable' release.

Debian 6 is 'Squeeze', which is now 'oldstable'. That said it's far from EOL yet and will continue to receive updates for some time (> 1 year).

If you're wanting to roll out a new production system *now* Squeeze *may* still be the best option, just because most people will only have been running Wheezy for a few days so there may be some initial teething problems. That said it's been in release freeze for about 11 months while preparing the release, tracking down and fixing the major bugs so it should be pretty stable.

Any new development you're doing that's not for immediate rollout should probably target Wheezy, since you'd have to upgrade to it when Squeeze goes EOL anyway.

-Steve