I have had the first version of the tamron 17mm, I currently have the Sigma 18mm 3.5 that has been fitted with a 72mm filter ring.
I have compared the Sigma directly with the Nikkor 18mm 3.5 one after the other on the same roll of film. The Nikkor is ever so slightly more contrasty than the Sigma and hs slightly better resolving power. As far as the Nikkor 18mm being a distortion freak, I don't buy that.
Essentially none of these lenses work brilliantly wide open, they all need some stopping down.
From around 5.6, the Nikkor is pretty good, from f/8 onwards it is very good and from f8 and a half it is brilliant.
The Sigma is not as good until f/11, but the differences are minimal and with the price being one third of the Nikkor lens, I bought the Sigma about 27 years ago.
Mick.