Or you could all actually answer his question ;/
The betas are there for us to use them, and we should be encouraged to submit bug reports of what we find goes wrong. But to say that "If you don't use the betas only for testing, you're a horrible person" is downright absurd. Just as it would be absurd to say "You aren't allowed to use Firefox unless you're one of the developers."
It's a basic fact of public betas. There will be people who just want to use it because they need the new features it provides for some reason or another. If they find a bug in their everyday use, hopefully they'll submit it. And if they do, then the program is made better by just that tiny little bit. This is why there are public betas in the first place: You attract more eyeballs. Just letting people go about it in their every day use and reporting problems when they run into them is how around 70% of bugs get reported in programs with public betas.
As for your question, use the latest one, as it will (logically) have the most bugfixes and thus, will be the most stable. There aren't any more features being added (aside from various things being finished up that were only half-implemented before the feature freeze) so you don't have to worry nearly as much about new bugs popping up that get in your way.