Synn:
I have both, and each has considerations.
The Zeiss, is a great lens, but can't begin to pull the same hyperfocal distance of the 14-24, just can't. It's going to vignette pretty harsh wide open also. I find that by F11? or so you can get a pretty good hyperfocal distance. The Bokeh to me is not that great either, not smooth. Plus to this lens, it's all metal, and has an excellent manual focus dial. Basically you can just turn it to infinity and you are done (unless you are working on a selective focus). LR cleans up the vignetting perfectly and on the D810, you won't pull much noise there either. The 18mm is smallish and light weight and I tend to carry it on field trips an I use it on all my night shoots, as there just is no coma aberration wide open. CA is nominal also.
The Nikon 20mm 1.8, well, it's coma prone until you get to around F2.8, (at least mine is), I picked up this lens mainly for astro work. The lens is not a great manual focus lens but that's true to me of most of the nikon prines with AF. The Hyperfocal distance is better than the 18mm and by F 7.1 or so you can get 18 feet to infinity. The 20mm is all plastic, typical of the newer Nikon glass. It's light weight however and will get the job done. Bokeh to me is harsh, nothing like the NIkon 24 1.4 which has excellent bokeh but some of the worst coma I have ever seen.
Both lenses are good considerations, but for detail, I would give the edge to the Zeiss. BTW, the Zeiss 21mm F 2.8 is also a great lens, but I found it's hyperfocal distance to be problematic as the 18mm Zeiss.
The only issue I have with the Zeiss, for that matter all Distagons, the extremely cheap lens cap, which will break if dropped just once on rocks.
Edit: I would also not count out the Samyang/Rokinon/Bower 14mm. Very sharp lens, and bargin priced. Focus is a bit tricky but once you get used to it, results are very good. Also pretty much Coma free wide open, which really is impressive.
Paul