The question in my mind is this: Given it'll be manual focus, why not just get the Nikon 14-24mm f/2.8 with an EF adapter, and have the extra flexibility of the zoom? From what I can see, it's the best ultrawide in existence, prime or zoom. I suppose only thing that the Zeiss has in its favour is lower weight and the ability to accept filters. But apart from that, why would I want it, if it isn't significantly better?
The Nikon, while an excellent lens isn't all roses. The distortion at 14mm is extreme; not really acceptable for architecture (it isn't so bad at longer FLs though, but if you want 14/15mm you're SOL with the Nikkor). A prime like the 14L II does much better, the Zeiss should be even better.
The Nikon also suffers from pretty heavy flare, while the Zeiss should be pretty flare resistant given the performance of it's 18/21/25mm brothers.
The Nikon is the best UWA zoom in existence in terms of resolution, but primes like the 14L, 17TS-E or the new Zeiss still have advantages in their specific focal lengths. Of course, the Nikkor is cheaper than any of the three aforementioned lenses, so that's a nice perk.