I suppose choosing between AROS and the PPC systems depends on what the user wants to do. MorphOS on a Pegasos is a pretty satisfying replacement for a classic Amiga as far as user experience is concerned, being able to run a lot of the old 68k software as well as MOS-specific applications. I don't believe AROS is anywhere near that stage, but please correct me if I'm not up-to-date on that.
-- gary_c