Both lens are not in stock locally much. (The nikon 20/1.8g was released awhile ago, but stock has been low. The Sigma 24/1.4 ART is not out yet for Nikon)
I've owned and used the 20/1.8G and would say its a very sharp lens in the center, minimal distortions, vignettes a lot, stepped down its good across the frame. Worthy of a D810.
The question I have for you is, why do you want a 'wide' lens ? Architecture ? Portraits ? Landscapes ?
Other lens you can consider Nikon 18-35/3.5-4.5g (sharp, cheap, flexible, lighter), Nikon 14mm/14-24, Nikon 28/1.8g (very good lens, also cost effective and light), and oh yeah, the 16-35mm.. has VR, wider, bit pricey
Different focal length at wide end gives you very big difference in field/angle of view. Perspective is also very different. Basically the pictures coming out of your camera using these 2 lenses will be very different.
Focal length is what make different lenses so different from each other.... Slight aperture difference does not make any significant difference at so wide a focal length. what other differences are you looking for?