screen advantages: ships with Mac OS X, and most Linux distros. Can run against a serial port, so you don't need to install a "terminal emulator" program, all of which tend to suck more than screen.
tmux advantages: much better default setup than screen. Ships by default with OpenBSD. Ctrl-B key doesn't conflict with as many default keystrokes as screen's Ctrl-A.
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screen advantages: ships with Mac OS X, and most Linux distros. Can run against a serial port, so you don't need to install a "terminal emulator" program, all of which tend to suck more than screen.
tmux advantages: much better default setup than screen. Ships by default with OpenBSD. Ctrl-B key doesn't conflict with as many default keystrokes as screen's Ctrl-A.