1. tmux does not require suid root for multi-user. Even if you solve all of the permissions issues for screen, there is an explicit check for uid == 0 when you attempt to enable multi-user.

2. tmux has vertical splits. Yes, screen has these, but they require you to patch/build your own screen. I've yet to come across a patched screen that has vertical split support.

3. tmux can integrate scrolling with iTerm2 (IIRC, there may also be some urxvt integration, but you'd have to verify that).

4. In tmux, splits are first-class citizens. You can do "C-b <space>" to switch between 'layouts' much in the way that you would in a tiling window manager like xmonad.

5. tmux remembers your splits/layouts when you detach/reattach. Screen doesn't last I checked.

6. tmux has a 'show-messages' command that has a log of all messages that it showed at the bottom, so that you can view them if something popped up that you missed.

7. I like the config format better. I was able to convert my highly modded screen config to tmux in an hour or so, and the config file is much smaller/simpler.

8. When scrolling in copy mode it tells you where you are in the scrollback buffer (e.g. "0/32").

9. tmux has this (from the manpage):

  has-session [-t target-session]
                     (alias: has)
               Report an error and exit with 1 if the
               specified session does not exist.  If it
               does exist, exit with 0.

10. Also this:

  respawn-window [-k] [-t target-window] [shell-command]
                (alias: respawnw)
          Reactivate a window in which the command has exited (see the
          remain-on-exit window option).  If shell-command is not given, the
          command used when the window was created is executed.  The window
          must be already inactive, unless -k is given, in which case any
          existing command is killed.