The 200-500 has a great ratio price/IQ, but for some it's just not enough.
I have used the 200-500 shortly so no big comments, but was surprise Brad prefers the Sigma, not because is a third-party but knowing his taste for fixed aperture lenses. I have tried also the Sigma, again shortly, but didn't bring any bells, neither one.
I praise Brad Hill's field observations, and from his writings, he's someone that knows what he's doing. So his findings are a great source of info for wildlifers.
But around here we also have great photographers that test lenses. Not brick photographers but real life testers.