I think that the differences between the images produced by a slew of renderers are often subtle. And the ‘best’ image is often a matter of personal preference.
But I have a kind of Turing Test for renderers: if I can’t tell whether I am looking at a render or a photograph then that renderer passed the test. Some years back, when I was choosing a renderer, my choice was governed by the work on display in each company’s gallery. I bought the renderer with the gallery showing the most photorealistic images of things I might want to render.
Of course, other factors like plugin availability, price, support, workflow, ease of use, material libraries etc matter too, so my choice has not been without angst.
Would I have bought a renderer if Rhino had cycles back then? Probably not. I’d not have got quite such nice images, but the difference would have been so slight it would have been comfortably offset by the extra beer money in my pocket.
But do look at the images in the galleries (at full size) and see what floats your boat (pun intended).