Both produce great quality images. Paul (AnimalExplorer) has posted quite a few images taken with the Nikkor 105/2.8, see for yourself. The Sigma also produces great quality images (this image always stands out in my mind). The choice for me would be between focal length required and compatibility. I'm in the process of selling my Sigma lenses and replacing them with Nikkor, just because I've had so many problems with two Sigma 50-500s. So for me probably the Nikkor, although I'd be willing to try the Sigma. Do you have an option to try both lenses before you buy?