I don't know if you do a lot with region/pane layout. In both tmux and screen, you can split panes/regions into sophisticated layouts. The difference is that tmux remembers it when you detach and later re-attach. Screen will forget the layout as soon as you detach. This can be pain if you use it for development and monitoring.
Some would see that behaviour as a disadvantage of connecting to the same session: with "screen" multiple people can connect to the same session, but have different, independent views. With tmux, all is shared within a session.
To emulate the screen behaviour in tmux, you have to tell tmux to start a new session in the same session group as the original session. See the man page for tmux, "new-session -t".
I don't know if you do a lot with region/pane layout. In both tmux and screen, you can split panes/regions into sophisticated layouts. The difference is that tmux remembers it when you detach and later re-attach. Screen will forget the layout as soon as you detach. This can be pain if you use it for development and monitoring.