I have the 35/1.4, which I love, and just picked up a 35/2.8 -- partly because I got a great deal on it, and partly because I figured it would be useful to have a more portable alternative to the 1.4, especially as I love the 35mm focal length.
I've done some comparison testing between the two at f/2.8 and I've been surprised to find that a) they seem extremely similar, save for a little more vignetting with the 2.8, and b) my 2.8 seems like it might actually be doing a hair better than the 1.4 in the corners and edges.
Now obviously I bought the 1.4 for the larger aperture and the gorgeous bokeh, which the 2.8 can't touch, but I'm a little surprised at the result. I do need to do some more testing, especially as very subtle differences in focusing can likely translate into noticeable differences when pixel peeping an A7rII image at 2:1.
But I was wondering if anyone else had done the same comparison, and whether the conclusion was similar or different. DXO suggests that the f/1.4 should do a little better than the f/2.8 at f/2.8, making me wonder whether I might have a suboptimal copy of the 1.4 (or maybe a great copy of the 2.8!).
Edit: I forgot that Amin had posted great comparison shots at another forum. His samples do seem to suggest that the 1.4 should do a touch better than the 2.8 when shot at 2.8...