In i3/sway, windows are organized in a tree structure where application windows are the leaves. Non-leaf nodes are called containers and can be tiled vertically, tiled horizontally, stacked, or tabbed. Windows and containers can be either floating or tiled on the workspace. It seems like i3's layout model is a superset of wmii's.
wmii's tagging does sound unique if I understand correctly that you can have 1 window in multiple workspaces and correspondingly in multiple positions and sizes. Something like that sounds like it would be problematic for handling multiple monitors, though. If one window is to be in 2 workspaces, what's supposed to happen when you want to show both workspaces at once in separate monitors? Can't show the same window in 2 places at once with different sizes.
A wmii tag workspace encompasses all the monitors; if I switch tags on one monitor, it switches tags on all monitors; you can't show multiple tags at once.
In principle, wmii handles multiple monitors great; it's had Xinerama support the entire time I've used it (checking mercurial, since late 2008; first being included in a stable release in late 2009). Though, I've been discovering that it has some bugs with layouts that aren't just the monitors tiled horizontally. (That doesn't stop me from using it that way, just that there's some bugs for me to grumble at.)
wmii's tagging does sound unique if I understand correctly that you can have 1 window in multiple workspaces and correspondingly in multiple positions and sizes. Something like that sounds like it would be problematic for handling multiple monitors, though. If one window is to be in 2 workspaces, what's supposed to happen when you want to show both workspaces at once in separate monitors? Can't show the same window in 2 places at once with different sizes.