All three NG OS' have a lot of specific difference.
To me all three feel very Amiga-like.
And the poster that said AROS felt the most like 3.1 was dead on (but is that necessarily a good thing).
I must admit, thought I can see the potential of AROS, I'd still prefer the PPC variants.
Since Hyperion has the right to include all of 3.1 in 4.1 (and beyond) compatibility may be better with their product.
But my own choice (which i won't belabor) is the most polished and the better performing of the two. Plus Ambient just feels better to me than any other interface.
I don't miss Workbench, and Scalos was interesting but not quite what I wanted, so I'm glad I have ambient.