I have read the Nikon has a bit wider field of view, 14mm while the Sigma is actually 15.5mm. Sigma glass looks to be of very similar sharpness in video reviews; am I in reality gaining anything upgrading from the Sigma I presently own to the Nikon? Sigma images certainly appear sharp when lens is closed down a couple stops..

Amazon has the Nikon on sale.

Another question. Amazon lists the Nikon as a Japanese market model, so should I surmise it to be Grey Market with no US warranty?

ETA. D850 and D810 bodies.

You might want to peruse this thread.

https://www.ar15.com/forums/general/Nikon-lens-opinions-sought/121-2399774/

Yeah, it would be a grey market lens.  Nikon just had refurbs on sale btw.

The Nikon is sharper when wide open, as well as a touch faster if you're doing extreme low light photos.  Honestly, they are pretty on par once they're stopped down though.  Sigma has slightly better coma and corners, but the Nikon has better color, slightly wider, and 1/3 of a stop faster in practice.  

Personally, it's splitting hairs between the two of them.  I went with Nikon and am very happy with it, but the Sigma didn't even exist when I bought one.  I'd have had to do some hands on work and agonizing to be honest as they're both great lenses.

I did read this thread initially, and a couple of times further.

Amazon has the Nikon well priced currently, so just wanted to inquire further of others' experiences.

Appreciate your expertise. You have saved me some expense since you note the lens' being very similar in quality once stopped down a couple of steps. (mostly I do nature and landscapes)

Grey market I tend to shy away from: not that in 30 years I've ever had to utilize a Nikon factory warranty. (the Amazon discounted pricing and "Japan market product" made me surmise grey market.)