Which version is the most stable ?

support requests aren't critical.

the natural progression of code is:

-dev
-alpha
-beta
-RC
-stable release

Thus you shoud use the beta release and not the alpha release.

It's worth nothing that while panels 2.x is a stable release, it is considered "fundamentally broken" in several unfixable ways by merlinofchaos, the author. So, even though it's still in beta, 6.x-3.x is a better bet.

There is no alternative module that does what panels does exactly. though you can look into the composite layout.module.

as far as reccomended versioning is concerned, I disagree. the beta is reccommended because it is the living version for Drupal 6.x, thus if you are going to test (again it's a beta release) than you should test the 3.x version.

alphas and betas shouldn't be used on a production sites by those who can't fix bugs on the fly and even in those cases the risk is the deployers. RC = release candidate and at that point you are gettin close to a stable release and as such bugs found are most times small.

The tagging of alpha, beta and RC has been around a long time and is in use by many projects, some in the computer field and some not. It's pretty standard these days I believe.

tommytom: Stop wasting my time and go use some other module.