tmux is like that thing that you hear about quite often, that you think it's not that useful, but that, once you try, you realize why and where its fame comes from. I adopted tmux about a month ago and I can't really complain much. It's just awesome. If you work a lot in the terminal (I do), I would highly recommend using it.
I did not switch yet. I use screen often, and each time I checked the diff with tmux it seemed to be the same thing with slightly different defaults.
Well, if you use screen and it works for you, then it might be a good idea to keep using it. Why change something if it works? :D. Both tmux and screen are similar. By the way, that's the config I personally use and it works pretty well: https://github.com/tony/tmux-config.git.