There are four "channels" for Google Chrome; stable, beta, dev and canary. Canary is equivalent to nightly builds AFAIK. As long as you don't disable Chrome's auto-updater, you're staying up-to-date with newest builds.
So the question really isn't "which beta", but which channel you use. I stay with stable, have tried beta occasionally though. Latest stable is 15, which would make beta 16 and dev probably 17.