Re: TEST IT
I was shooting two systems, where I had the Sigma 35 ART for my D810 and the Sony/Zeiss FE 35mm f1.4 for my A7R. I converted to all Sony, so I had Sigma convert the Nikon mount to Alpha mount to use on my A99's.
I recently did a round of testing of both lenses on the A7R (using the LA-EA4 adapter for the Sigma) with flash and a tripod. I realize that DXO rates the Sony lens at 23P and the Sigma at 28P (or 29?), but in looking at 200% crops on a 5K Retina monitor, I can tell you that at f1.4, one would be hard-pressed to identify a significant difference in sharpness. The Sigma is just a hair sharper. Looking at anything less than 100% crop does not reveal any measurable difference. Any difference is likely due to a bit more contrast, wide-open, on the Sigma. Both lenses are amazing.
The Sigma has slightly less CA's, but more vignetting. The depth a field is noticeably narrower with the Sony lens. This actually makes it more difficult to focus on near objects when wide-open. Maybe this is what people are talking about when they talk about "Zeiss 3D".
The major difference is the bokeh. The Sony is a cream-machine, while the Sigma is a bit rougher with slightly nervous bokeh.
If I were to do it over, again, and as much as I love the Sony bokeh, I would probably just buy the Sigma, as it is a lot more cost effective and delivers terrific results. Also, the new A7RII will allow the use of the LA-EA3 (no mirror), so the native camera focusing system can be used with adapted lenses.