I see it this way:
AmigaOS 4.x : It is a an easy transition from AmigaOS 3.9, everything is familiar. It has the "Amiga" name. But it is still pretty immature, as it seems to be a victim of constant crashes. Anyway, Hyperion seems commited to fix them in the long run.
Still it has lots of quirks such as no USB 2.0 suport, something which any decent semi-modern OS has. It is propietary and closed source, meaning it depends entirely on one Company, and if that company dies, then that is most likely gonna be its end. It has a high price tag and is tyed to an old underpowered PPC architecture which is hilariously expensive.
AROS : It is not a straight-thru transition from AmigaOS 3.9, but nothing an average user cannot do. It is still immature, and was a once stagnating project, that it is now progressing very quickly. It has really good USB 2.0 support. It is open source and freely available, it is married with the cheapest and fastest architecture we have today, x86 PCs (But it can be divorced from it if needed).
Speaking about emulation, OS4 emulation is a little bit flakey, programs that hit the hardware, which means most games and demos, will not work, or at best work with issues. AROS emulation it is far from perfect, as it is not completed, but it is really good. I suggest you see it for yourself download the latest IcAROS distribution of AROS, and try it out. It is without a doubt better than OS4.
Performance wise, AROS beats the hell out of OS4. Its architecture gives AROS that freedom and power.
Bottomline: I am not biased by the love for an OS or the hate of another, it is what is. And still it is great to have them all.
Technically speaking OS4 still leaves a lot to be desired, but then a human choice on an OS, it is rarely based on its various technical aspects, but rather on the subjective personal experience/desires/feeling they see/get that they will particularly perceive as fullfilling in that OS.
As a sidenote, there is also something that needs to be clarified: The Amiga hardware and its AmigaOS are both long dead since about the time of both the Amiga 4000T and AmigaOS 3.1. All we have here, name it Pegassos, SAM, or even an AresOne, are "Amiga-like" hardware and that happens to be the same with MorphOS, AmigaOS 4.x and AROS. It is not that they are bad or evil, it is that they are not Amigas or AmigaOS, nothing against them, just "Amiga-like". Please dont be fooled by cheap marketing or passionate advocates.
My 0.02
