I think both systems are reasonably good, despite they have differences. However i do choose MorphOS, for a handful of simple questions.
Hardware supporting MOS is faster.
Hardware supporting MOS is cheaper.
People developing soft for MOS seems to be more serious... (but this is a particular thought)
However on the other hand it seems there are more movement on the OS4 side about, programs and stuff... But the number of Sam's sold is really really low. About if OS4.1 is worth of the Amiga name, i would say a "why not?" On the other hand after the wise move of MOS supporting mac hardware, i bet we will see more programmers and new stuff to come... And grow in number much more than OS4 stuff...