Hmm, yet another one of these threads...
OS4 can't be easily compared since it isn't particularly available to compare, as it is still in development, AROS can only run AmigaOS/x86 binaries, and so MOS kind of wins by default at the moment.
I haven't seen MOS in action, but anyway. I've used both AROS and OS4 for very short periods, and they seemed like they've made good starts.
