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].