you probably have good reasons to go with Heliar 15, but I wouldn’t rely on those comparison images solely.
When you want to compare lenses, you should consider many things before you proceed.
First – you should always compare apples to apples – 1mm on the wide side means quite a lot. Commenting on corner sharpness on tree leaves (favorite testing target of Mr. Rockwell, if I am not mistaken) and in general on any 3D scene (like they used to have, and actually still have on DPreview) is laughable at best. (But I am doing the same from time to time, because people likes it :-))
On such a complex scene, you compare center zone, mabe even midzones, but corners could be heavily affected by even light difference in focus plane, by DOF differences between two lenses (when you compare 15 to 16 e.g.) and also by different field curvature characteristics (at f/8 you should know your lens and how to focus it properly).
In my testing of Sony FE 16-35/4 G OSS, I never experienced that kind of corner smearing at 16mm as he is showing on his crops.
Could it be bad sample? I don’t know… User fault is certainly more probable IMHO…
BTW, last time when I spoked to Mr. Nasse from Zeiss, he showed me his lab testing of possible focus plane misalignment. In a result, if the test target was improperly focused by only 0,5mm!!! MTF of tested lens dropped over 10%, (even more in the corners). What does that tell us – it is far more important to properly focus the lens, than to pick the sharpest lens on the world. (If critical sharpness is important to you).
On the other hand, on the links you provided I have noticed much more pronounced purple fringing on Voigtlander. Could be that Sony is software corrected in the camera, but I think it’s rather optical correction in this case.
Most important however, is how you feel about it. If possible, I would suggest to rent both lenses and see how they work for you.
In my case, any of those would be more than perfect (except for that filter modification stuff), but flexibility of the zoom range, built in AF when needed, additional stabilization and partial weather sealing, will certainly shift me toward Sony.
However, I have SEL 1018/4 and A6000 and this is great system for most of my landscape needs. When I shot wide for my clients, I usually rely on Canon TS-E17, 24 and EF 14/2.8II.
Few “landscape samples of my A6000 + SEL 1018 combo: