It's easy... rather than the Ctrl+A key sequence that you use in screen, use Ctrl+B in tmux. And to reattach, rather than screen -r, tmux attach does the trick. That's 90% of the usability difference.
People usually point to the BSD license and better code-base as the main selling points. Then they point to vertical split (which you can patch screen to do).
Bottom line is, if you have a screen config you like - there's not much point in switching. If you're just getting into terminal multiplexers, it's worth a look.
