Stable builds are based on the latest CVS and include new features that need to be tested. Xt builds are also based on CVS, but are the result of ~1 year of taylor working on a separate codebase. The Xt builds tend to run smoother, faster, and also include normal mapping, which makes everything look a lot better.
If you just want to play the game, I'd suggest Xt builds. If you want to actually use a new feature, you'll probably have to use stable builds. Generally people mention in the first post of the thread which feature that a build has.
So
Xt builds = Better graphics, smoother gameplay, new features by taylor
CVS builds = 3.6.9 graphics, slightly jerky gameplay, new features by other coders