Do NOT use 3.0, it's pure crap. There is a reason it was only released for about 2 weeks. It's worse than 4.0. Leave the plane on 2.1, it's better than 3.0 indefinitely.
If you are flying them and having success, then you can stick with it. 2.1 is a great firmware, better than the other two I think. What matters is that you are successful in flying them.