For me, tmux has saner options and more mnemonic default keyboard shortcuts, that do not interfere with default bash shortcuts (Ctrl-A is goto-home-of-line in bash/emacs, but it is hijacked by screen by default as its escape sequence; tmux uses Ctrl-B for the same by default, which in the emacs bindings means go-back-a-character, and I do not use much, and is less likely to hit by accident).

I'm also wondering about this. I've been using screen for almost 20 years, and wonder if there's a compelling reason to switch.