I assume you had some quality time with the 200-500VR comparing it to your Tamron in Scotland, didn't it perform as expected?
If the Nikkor performed better than the Tamron it should be roughly on equal terms as the Sigma, it its worse than the Tamron the Sigma is the clear winner
The biggest gripe I had with Sigma was back focus issues, I assume the D500 can now mitigate that?
The 200-500 performed better than the Tamron, no question about that.
At close distances the difference was not that big. The color/contrast, AF and VR are better on the Nikon
Main advantage of the Tamron (and Sigma) is the extra 100mm.
From what I've read the Sigma Sports is more or less the same as the Nikon in IQ, AF.
Both can take a 1.4x TC. Both are F5.6 at comparable focal length.
Sigma is weather sealed and build like a tank (including the tripod ring/foot). Nikon balances better.
Prices are the same over here. There's a discount of the Sigma with TC at the moment (saves €150,00)
Weight is not a real issue for me, because when I travel I take the 300PF with me.
For both I need a bigger monopod though.
Another nice thing with the Sigma is the dock. In combination with the new AFFT feature of the D500 I expect no problems in this area.
I already have a seller for my Tamron, so I'll probably decide this weekend (if the weather is fine I'm heading to Texel the week after for some birding
).