I guess you have seen some of my tests here with UWA lenses on sony A7/r – http://www.verybiglobo.com/sony-alpha-a7-ilce-7-and-sony-alpha-a7r-ilce-7r-part-8-a7-ultra-wide-lenses/
Since that article, I’ve been testing few more UWA lenses, but nothing bellow 16mm. I don’t have experience with Voigtlander 15, but I have their 21/1.8 which also have built in hood and I basically don’t use it mainly because of difficulties with a filters.
Samyang 14/2.8 is best value for money, very sharp across the frame, and distortion can be noticed mainly on architecture. There are some profiles that are supposed to flatten its rendering (PT lens e.g.), but I simply never used it on architecture.
In my humble opinion, your best bet is certainly Sony FE 16-35. The flexibility of the zoom at the wide angle is even more significant than on most tele-zooms and image quality is excellent. Recently there is lot of talk about QC issues with Sony lenses, but either I was lucky, or not enough picky to discover problems with any of my Sony lenses.
If you can live with tighter framing, expensive but from what I have seen, superb (technically) rendering has new Loxia 21/2.8 (very pricey though), and you might take a look at some other options – namely Tamron 15-30/2.8 or Canon 16-35/4. Both options will be slightly cheaper than Sony FE 16-35/4. (Tamron has bulbous element too).
If you just need UWA, I will take a risk and try one of Samyangs 14/2.8 (I think they are developing also 12/2.8, but not sure about that), because it is fairly cheap and if you don’t like it, you might lose some 50 GBP or so, by selling it later.
If you want lens that will use all native benefits of AF and built-in software correction, I would suggest FE 16-35/4, despite its size and weight…