[0wned]

I use both. The only big difference? Besides the license? Screen uses 'Ctrl+A' while tmux uses 'Ctrl+B' and screen -r (to reattach) is replaced by tmux attach. That's it, for me at least. tmux seems to have a lot more features, but I only use 10% of them.

[silentbicycle]

The biggest difference as far as I'm concerned is that it has vertical screen splitting and xmonad / dwm-style automatic tiled layouts built in. I've also looked into the codebases for both, and if I were working on new features, I would far rather work on tmux's - it's much cleaner.

[graywh]

An easily changed option is not a big difference.

From what I can tell, screen has better handling of colors. tmux seems to mess up the <Esc>[90m; -like sequence for colors 8-15.

Also, I like the ability to highlight screen's current screen with colored parentheses.