I won't say anything about OS4 because pretty much everything I'd say would be in the con side and I don't want to pollute this thread with perceived bashing.

MorphOS
Pros:
Can be used largely as a real life modern computer. Use OWB for a few days, then switch over to OSX 10.58 and try to use TenFourFox! It is truly stunning how god awful and slow it is even on a 2.7Ghz G5. My g5 with MorphOS is the "modern" computer in my retro workshop. Development is seemingly the best given it's size. We all know most of MorphOS' strong attributes.

Cons:
We still haven't migrated to x86-64. Apple Mac hardware is truly getting old at this point.