Adding more experiential knowledge for reference:
The Zeiss ZM Distagon Biogon 35/2 is an outstanding lens, and quite reasonably priced vis a vis the Leica product. I have not owned it, but have seen hundreds and hundreds of shots made by a friend on Leica M9 and M10. The ZM 35/2 has very low distortion, as is common with M-mount 35mm lenses. Also it has surprisingly good sharpness at f/16---most lenses soften a bit at the very small stops, but the ZM 35/2 seems to soften a little less than most. Many of the Zeiss ZF lenses also share this property, in my experience.
The classic Nikon 35/1.4 K/Ai/Ais is much written about. At the risk of repeating what everyone else has said, my experience is that it has a unique, soft "dreamy" aberration look wide open, which gradually goes away as it is stopped down. By f/2.8, it looks 'normal', and the lens is famous for it's extremely subtle, gentle transitions from in- to out-of-focus. I also chose to keep the Zeiss ZF Distagon 35/2 over my copy of the Nikon 35/1.4 AiS back in the 12MP Nikon D3 days.
Added Note: Most all M mount lenses, both Leica brand and the others , have a close focus limit of 70cm. I find wide angle lenses that don't focus close to be undesirable; YMMV, as always