There aren't a lot of lens choices in this band for full frame. The Nikon 14-24 is probably the king. The Zeiss 15mm F2.8 is probably a tad better but not a zoom and no autofocus (no AF on a 5D3 either with a Nikon 14-24).
I got a Novoflex adapter for Nikon lenses on a Canon body. It is very well made, has aperture control and there is a chipped version for metering data.
The Samyang 14mm I have never used but I have 2 Samyang lenses. They are very good. Although I would thoroughly test the performance of a Samyang lens within the return policy period. My first 24mm F1.4 was decentred and had to be replaced. The 2nd copy is great.
Canon is working on a 14-24. No doubt to fill this gap in their lens lineup. When it comes out I am sure it will be a fabulous lens. It has to be, as the Nikon is close to perfect. I also expect it to be very expensive.
I have seen comparison images and corner performance between Zeiss 15mm and the Nikon 14-24. The Zeiss is better in the corners wide open but stopped down a bit the Nikon and Zeiss are the same or the Nikon slightly better.
The Nikon also outperforms a Zeiss 21mm F2.8 overall as well. No CA, sharp to the corners and a zoom (I tend to only use it at 14mm anyway).
So not a lot of choice in this focal length at the moment.
In a way nightscape images require different performance criteria than lense are usually judged by.
You want F2.8 (lenses under that I have not found to be usable before F2.8 anyway - Samyang 24mm F1.4 excepted - F2 is fine). You want aroud 14mm. You want no chromatic aberration. You want no coma in the corners. That's a tough call to be sharp wide open, no CA, no coma. That's one hell of a lens that can do all that.
Another choice is the Tokina 16-28mm F2.8. I don't know much about this lens but it may be worth considering.
http://www.dpreview.com/news/2010/02/22/tokina16282pt8
Greg.