As promised, here is a side-by-side comparison of the absolutely wonderful Sigma 35mm f/1.4 Art and the well-respected Nikon 35mm f/1.8 DX on a cropped sensor camera. What I found amazing is how close the two lenses are in sharpness. The bokeh, distortion, fringing, chromatic aberration and vignetting are clearly better in the Sigma but the Nikon really does hold its own.
Do I regret buying the Sigma? Absolutely not.
Below is cropped in from the above image.
The Nikon definitely shows more fringing in extreme contrast.