Andrew Alexander has the following short comparison conclusion on both lenses:
The Sigma still holds up well against the new Nikon: both lenses are very sharp. Where the Nikon is just slightly sharper at wider apertures, stopped down, the Sigma achieves just slightly sharper performance. In this case, "slightly" is measured in the tenths-of-blur units. CA performance is much better in the Nikon (especially at 10mm), corner shading is about the same, and distortion is perhaps a bit more forgiving in the Sigma. Both take 77mm filters, but the Sigma costs half as much. The Sigma 10-20mm is also newly available in a constant ƒ/3.5 configuration.