In Xmonad, I have configured twenty different workspaces, and I can easily swap them in and out of any given monitor. With a triple-monitor configuration, this is quite handy.
Xmonad seems to not get in the way as much as awesome. For my purposes xmonad is just a lot simpler, but I am sure I am barely scratching the surface feature wise.
A lot of the awesome docs/screenshots reminded me of enlightenment-porn back when e17 seemed like vaporware. I don't need crazy graphs/apps/eye-candy just help me arrange organize my windows and go away.
XMonad _is_ simpler, since awesome comes with a taskbar, system tray, and clock by default.
If I upgrade from XMonad 0.9 => 0.10, I don't have to completely redo my config file, because a good portion of the API has changed.