I'd go with a 16-35 if you can afford it. I prefer the zoom for focal length options, and the 16-35 is significantly sharper than the 17-40
I have a Canon 16-35 f/4 L IS and a Rokinon 14 f/2.8 (the old manual focus version), and the Canon was my favorite lens on my Sony A7 (on a metabones adapter). The autofocus was pretty bad, so I manually focused most of the time; AF on the newer Sony bodies (A7ii, A7Rii, A7Riii, A6500) should be fine. Pretty much the only time I used the Rokinon was for astrophotography, where the extra stop in aperture helped. For landscapes I was usually stopping down from f/4 anyway.
The native Sony 16-35 f/4 will be a little smaller (the lens is about the same size, but you don't need an adapter), and similar in image quality (I think the Canon is a little better, but they're close). If I was buying now I would get it instead of the Canon (when I bought the Canon lens I was still using a Canon DSLR as well)