It's a preference thing. I own both MOS and OS4 machines, and love both OS's for what they offer.
But there's still some things I am more comfortable interface and user experience wise with OS4 vs. MOS, and all the skinning in the world won't change that. Think of it as how people prefer their morning breakfast rituals - some people take their coffee with cream and sugar, some like it piping hot and entirely black. I'm just more comfortable with OS4, but you couldn't pry my MOS boxes out of my cold, dead hands.
OS4 is not a better OS than MOS, or vice versa. Both are great experiences, but you'll likely prefer one over the other for strictly proprietary and personal reasons. It's the same reason why one person will buy a Chevy over a Ford, etc. It's a hobby, and it's a fools game to try and compare a hobby OS with a mainstream OS in terms of functionality. I for one am thankful that in the Amiga scene we have all the choice in OS's that we do.
I've not tried AROS for some time, but I hope it gets to the point where I can build a new PC and simply install AROS on the machine. As it sits, I lost a lot of interest in AROS when I had to build specific hardware builds to run it optimally. I built an AROS machine up, then just converted it to an Amithlon machine. That in itself doesn't make AROS a poor choice, it just wasn't for me, I suppose. I hope in the future AROS (native, non hosted) can support pretty much any commodity hardware, but as it sits I can't even begin to consider running it on this machine - it doesn't work at all on this particular hardware, but the devs are covering a lot of ground rapidly with the hardware support, so I'm expecting very good things out of AROS over time. It is quite a chore for them to try and support all commodity hardware, no doubt. That being said, I'm admittedly and vehemently against "hosted" OS experiences, and I am aware that the hosted versions support a lot more hardware than native does.