These days tmux seems somewhat more actively developed, this might be a subjective appreciation, though. But at least the tmux mailing list seems more active. I am subscribed to both of them.
The feature that made me switch from screen to tmux back in the days was vertical splitting, which just worked and worked well (not at turtle pace, not blowing my cpu like in screen). It happens that nowadays I no longer use that feature, but I kept tmux because I find it simpler to handle, because it's lighter in my RAM, and because a few problems with screen (besides splitting) that I no longer remember. I switched long ago. I have no idea if my concerns about screen still hold true. Not do I care, since for now I have no need to change back. -- Jesús Guerrero Botella
