I'm not going to go over what i said in the other thread, but in summary the Nikon is easily the best of the 2, but quite some margin IMO
Having said that, folk on that other thread seemed happy that their Sigmas were sharp at 600mm, and mine was never acceptably sharp wide open, the Nikon however is a sharp as a tack wide open at the long end
For me there is no contest that the Nikon is the better lens, i only went for the Sigma at the time as brand new the Sigma was considerably cheaper than the Nikon, but as there are more on the 2nd hand market now, i managed to upgrade from the Sigma to a 2nd hand Nikon for under £200, and there was a much bigger price difference than that when they both came out
One of the big advantages for me as well was the constant aperture, as i tend to shoot in manual mode for wildlife, so not having to worry about the aperture changing as i am zooming in/out is a big plus point
I suppose the only real drawback is the slight added weight of the Nikon, but it also feels substantially better put together