Have you used both? Because I have... the focussing on the Sigma is much faster in my opinion, and I don't think you'd be able to tell a difference in IQ, I couldn't. The Nikon was a little more contrasty perhaps, but the Sigma definitely holds its own.
I have to admit I have not used the Sigma II, but have used his older brother and own the 24-70 2.8 siggy. When I used the older version it focused about the same speed and felt like a fisher price toy compared to the Nikon which is pretty much one big hunk of steel and glass.
So since I have not used the newer one, I went looking for reviews. DPreview.com was happy with the lens although they did mention some issues with image quality such as:
Image quality when shooting at short focus distances with wider apertures is distinctly weak, with a combination of general softness, focus shifts due to spherical aberration, and red/cyan fringing around slightly out-of-focus regions (due to axial chromatic aberration) giving very unconvincing imaging indeed.
and...(emphasis is mine)
The optics are perfectly competent, if not outstanding; indeed the lens is really very good towards the short end, but weaker at 200mm, where it's slightly soft and suffers from relatively high levels of chromatic aberration.
From what I can tell reading reviews, and from remembering the older version, it also does not have weather sealing which the Nikon does.
DPreview does not review the Nikon so I went to other places. Fred Miranda lists the lens and it seems to have comments for both the original and II siggys showing 64% of the people would recommend it with an overall rating of 7.8. The Nikon is also reviewed these showing 95% of the people would recommend it with a overall rating of 9.4.
Then if you like Ken Rockwell (I like some of his stuff, don't like some of it, meh) he talks about the Nikon as:
This Nikon 80-200mm f/2.8 AF-D (new) is the world's best buy in professional telephoto zooms.
Instead of that, lets look at the #1 seller of photo equipment online,
B&H Photo. The Sigma lens you mention has 126 reviews, 4.5 stars, 8 people say it focuses slow and 5 people say it has blurry focus. The Nikon lens has 196 reviews, 5 stars, 5 people say it focuses slow (you would expect more since it has more reviews) and 0 people say it has blurry focus.
Now, add all that together and that is why I recommended the Nikon over the siggy. Not that the siggy is a bad lens, but if I was in that price range already, I would go ahead and spring for the Nikon. I bought my 24-70 siggy because the Nikon was $1700 and the siggy was $550 so I got 90% of the performance for less than 1/3 the price, that is not true with the 70/80-200 2.8 lenses.
Allan