I hadn't heard of Ratpoison before. There's also Xpra but I still haven't gotten around to trying it out. Unlike Ratpoison it's not a window manager so it should be usable with whatever window manager/desktop environment you like.
Who needs tabs when you've got Screen/tmux? :P
Check out the tmux FAQ for the main differences. Some of my favourite features include vertical pane splitting, automatic renaming of windows/titles, better UTF-8 support, the status bar being on by default. I also like that the default key binding doesn't break the shell usage of Ctrl-A to move the cursor to the beginning of the line, but bare in mind that a lot people hate it and end up remapping the keys to a layout similar to Screen (the prefix key in tmux is Ctrl-B). I prefer the key mapping for handling nested sessions too (press Ctrl-B twice). The memory usage is much better in tmux too!