I have and love the Sigma. It's a rock solid, super sharp lens. I bought it to save me some money because I'm of the opinion that the IQ is no different and have no regrets on that decision. When I bought my D750 I was forced to send it, my 24-70mm f2.8 and 150-500mm back to Sigma for firmware upgrades because each had an anomaly when used with the new body. Sigma does the work for free (I had to pay shipping there, they pay return), and it took less than a week. But it's an inconvenience. If I have one nit to pick on the lens it's that the stabilization system is quite loud compared to any other lens I've owned, but once you know it's supposed to be that way then you stop noticing.
I have and love the Sigma. But if I was buying now and could get the Nikon at only $100 more I wouldn't hesitated to spend the extra. Nikon seems hell bent on screwing with 3rd party manufacturers (when a friend experienced issues trying to get my Sigma 70-300 macro to work on her D7200 she contacted Nikon and was told, "Nikon recommends only using Nikon lenses on our bodies", and offered no other assistance in debugging her issue), so unless you're willing to accept that as part of the deal once you have it then stick with the Nikkor glass.