Which version of CAPTCHA to use in Drupal 6.9?

What's important is that it's Drupal 6 - so therefore whatever the latest version of CAPTCA is at a given date and time, that is the one you should use. Well, the one that's labeled "recommended". You can always try the others with any module, but "recommended" is always least likely to cause any problems.

The only part of a module's file name that associates it with a given version of drupal is the initial number - aka Module 6....... will work with Drupal 6.xx (6.1-6.9,6.10,6.11,etc)

Or if I'm wrong on this, please someone correct me - I've only been tinkering really with drupal for maybe 5 or 6 months, if I remember correctly. ;0)

I am the current maintainer of the module, so I hope I can give you some insights

The 6.x-1.x branch is a rather straightforward port of the 5.x-3.x version of the module to Drupal 6.

The 6.x-2.x branch contains new development of the module, with some architectural changes to cope with problems in previous versions (especially related to sessions/cookies etc).

For the moment I am the only maintainer of the module and do this only in my free time. I can't allocate a lot of time and therefore I only focus on the 6.x-2.x branch. The updates to the 6.x-1.x branch are typically additions of translations, there hasn't been and won't be a lot of work featurewise on that branch anymore.

For the more practical aspect of your question: both versions (6.x-1.x and 6.x-2.x) are functional and should work. As there is no interesting data storage by the CAPTCHA module (apart from the configuration settings), it is harmless to up/downgrade from one version to the other (for downgrading I would recommend uninstalling the module first).

For the moment I would still recommend the 6.x-1.x branch (e.g. 6.x-1.x-dev, which should not be less stable than 6.x-1.0-rc2),
but note that it is developmentwise a dead end.
The 6.x-2.x branch should become the recommended branch in one or two months (I hope).

I hope this clarifies the situation a bit for you?

Well, I suppose the maintainer(s) should reply to this one, anyhow. ;0)

sorry about that - was a clicker accident. ;0)