You have not told us what you will use it for.
IQ is important, not the subject.
The fact that Sigma is called Sports is irrelevant to me, as I will not be shooting sports, though I might try BIF, which is somewhat similar. Will even try to use it for landscapes and even casual shooting, or if you wish a walk around lens.
I sold my 70-300VR, that was pretty good upto 200mm, and now want a practical upgrade. Will never go for big guns like the 400/500/600 fixed focus lenses. Neither for the 200-400. They are just to expensive and unjustifiable for what I do.
The Nikon 80-400 would suit me just fine, except for its price. Sigma comes cheaper, so this is why I keep revolving around its IQ. If its a mach for the Nikon's IQ, there is nothing left to talk about.