Sigma EX lenses are extremely sharp, even wide open. What’s important to consider is color balance. Sigma have a traditionally yellow cast, where as Nikkors are neutral to reddish, but not as red as Canon. Tamron’s have a closer color balance to the Nikkors than Sigma. Tokina’s are slightly blueish in hue. Yes, you can filter it all out with PhotoShop, but that’s not the problem.
A pro wants consistent color balance and contrast across the lens line so pick a brand and stick with it for future purchases. I would suggest that the Nikkor will have better contrast at widest aperture, and will last much longer against abuse and daily use. Sigma USM focus is very nice but it will fail over time. Quality control will be better with the Nikkor and you can expect to return the Sigma once or twice to get the glass you want. Sigmas are known for sticking aperture blades over time. 10 burst exposures at f8 may yield 2 or 3 overexposures.
Sigma focuses in the opposite direction than Nikon. That alone would prevent me from putting it on that camera as I primarily manual focus for everything.
An excellent (and much cheaper) alternative would be the Tamron 28–75 which does focus in the same Nikkor direction. Look for a used one from a good feedback and get it for much much cheaper. Very light weight but some question its durability. It’s fine for regular use and will be much closer in color balance to the Nikkors. Highly rated optically.
If you are using a full frame Nikon camera, then the Nikkor lens will have sharper corners. But they will all be pretty equal on a half frame.
Tamron also has a matching wide angle 17–35 for full fram and 17–50 for half frame that also focus in the Nikkor/Leica/Pentax direction. Alas, the Tamron 70–200 focuses opposite in the Canon/Sony/Olympus direction. The Tokina 80–200 focuses like a Nikkor.
Check the zoom directions as well. They will change from camera/lens combos.
The Nikkor will flare less than the Sigma, Tamron or Tokina.