GNU Screen is a magical thing. Even after trying tmux, I prefer its interface. It's screen splitting behaves as I would expect -- easily allowing the split-portion to cycle through other open buffers (tmux treats split windows differently than "windows" and it sucks. And more/most importantly, if you were sharing your terminal with a partner, screen didn't force you and your connected partners to be locked into the same synchronized view -- you were free to navigate windows independently (as the GNU lords intended), while sharing the underlying session as a whole. It allowed concurrent, collaborative workflows (as seems to *not* be the case with tmux).
Tmux does have a major advantage, which was enough for me (to my chagrin) to switch. When your screen session itself becomes (not disconnected, but) corrupted *or* if you accidentally kill the terminal which was running screen (which I do constantly because I am an idiot) -- you then lose any windows and buffers associated with it. No ability to attach, just gone.
Tmux, follows daemon approach (like emacs-as-a-daemon) wherein a server runs in the background and multiple sessions can be maintained. I like to think of this approach as analogous to "chrome" which maintains tabs as separate processes (i.e. the analogy of tabs to tmux sessions as isolated containers). So long as the tmux program doesn't become corrupted (i.e. the daemon stops running due to segfault or pkill, etc) you will be able to reattach to the session.
I am also pretty sure tmux (like emacs/vim) can have these features from GNU screen I want, I just need to invest the time scripting them or submitting a pull request (as if I don't have enough projects) whose investment will no doubt be greater than the time savings and learnings.