The overall design to tmux is quite a bit cleaner, and (having poked around in their respective sources), I know which I'd be more likely to contribute patches to. It took screen forever to get vertical splits (are they still an unofficial patch?), whereas tmux already has dwm/XMonad-style layouts.
tmux is also included in the standard OpenBSD install. Make what of that you will.
Granted, my screenrc is a hotchpotch of cut-and-paste and default stuff from lots of places, but that's actually the point: I could read through the tmux manpage and set up tmux the way I wanted, without cobbling a conf file together from the blog posts and mailing list emails of wiser people.
tmux 0.8 (20090604-0818)
Edit: 1.1 package: http://ftp.us.debian.org/debian/pool/main/t/tmux/tmux_1.1-1_...
Some commands are still not working (pane-active-border-bg and capture-pane, it seems) but I can take a closer look at those after playing with it for a while and getting comfortable.