These lenses are both extremely similar. The Sigma focuses much closer than the Nikon which gives a magnification of 0.5X compared to 0.25X. This lets you fill the frame with a smaller object, roughly half the size with the Sigma.
Note that the Nikkor is lighter and does not have a built-in focus motor, so it will autofocus only with a mid-to-high-end camera. With entry-level models, currently D3xxx / D5xxx, it still works but only with manual focus.
Image quality is the other differentiation but it has been a while I've seen these lenses. The Sigma is a lower-end offering, so expect it to be of lower image quality. Sigma has improved image quality over the years, but I'd say that model predates this.