MorphOS is more responsive, plays HD videos better and faster and includes a nice HTML5 browser. Moreover, it allows you to run Amiga software (including PPC demos) at decent speeds, unlike OSX+uae. I can launch modern amiga demos directly (unlike OSX). MorphOS is supported and updated.
PS: I haven't benchmarked it but Poseidon seems faster than OSX transfering files through USB2. Perhaps RTCW is faster on MorphOS too.
You have ofcourse data to back up these claims? I really don't notice much speed difference between MorphOS and Linux on my minimacs, and Linux for sure has way more features that are usefull for me.