You're going to open a big can of worms with that one ;). Tmux has quite a few more features than Screen, with Screen having a few features that are useful in edge cases[1]. It's hard to argue with tmux having bindings for vi/Emacs, as well.
If you're on a mac and use iTerm2, using tmux is somewhat of a no-brainer, thanks to the integration between tmux and iTerm2[2].
With reflowing text added to tmux, and giving that "Lots and lots of bug fixes, fixing memory-leaks, etc." the benefit of the doubt, then I would say that unless you have a specific use case of screen (and if you do, you know it) then you should be using tmux.
After mastering screen, I switched to tmux. It's a lot easier to configure. It's also a lot nicer to split panes etc...
Definitely recommend the switch. You might be dreading it, but two days into tmux you won't regret it. It took many weeks trying to convince myself screen is better until I actually tried tmux out.
The only two pains that come with it are a) having to type `` to get a single `, and b) pasting anything containing a backtick behaves as if you'd typed it out.
If you're on a mac and use iTerm2, using tmux is somewhat of a no-brainer, thanks to the integration between tmux and iTerm2[2].
1: http://sourceforge.net/p/tmux/tmux-code/ci/master/tree/FAQ 2: https://code.google.com/p/iterm2/wiki/TmuxIntegration