The 2.4 version that is due to be released shortly is an updated version of the 2.3 branch which is stable. The 3.x is a complete rewrite is likely to be less stable. I haven't used 3.x for a project yet, but it does look interesting.

Here is a good discussion on the subject: http://forum.kohanaphp.com/comments.php?DiscussionID=3477...