There is a $600 premium for the Sigma Sport versus the Nikon. For that you do get a weather sealed lens with a small increase in reach.
Small increase?
You get a 150 mm increase in range.
You also get
the best bokeh of
any zoom lens under $4000.
And you get comparable resolution of any lens under $2000.
And no facts are in evidence about the Nikon 200-500mm in the linked post. From all reviews, it is optically very good with no light falloff wide open. And the VR is reported to be stellar if you need that feature.
We both are waiting for the formal test results.
However, because the Sigma pretty much out-performs every lens in the 200-400 mm lineup, and because the 200-500 mm Nikkor is so much cheaper than Nikon's 100-200 (which is at best "comparable" to the Sigma), I suspect the 200-500's marks will be underwhelming, and significantly less than the Sigma's, when all is said and done.
The only thing that is factual at this point is 200-500 is
less broad than 150-600, and it is
cheaper than other Nikkors.
In time I suspect that the benchmarks of the 200-500 will likewise prove to be underwhelming as well, which is why they priced it as they did.
And, BTW, the IS of the 150-600 Sigma is excellent also.
Focus speed, lock and tracking are real keys to these lenses and I have yet to find a 3rd partly lens that outperforms the OEM brand (Nikon or Canon) in this area. Obviously, I have not tested them all.
The real question is, have you tested any?
The Sigma's IS is quite excellent, and it blows away the Canons/Nikons in a lot of different levels, actually.
Please review
My Table for specifics.
Jack