Gpu reccomendations 2016 Nvidia vs Amd

Hi Guys,
I am currently searching for a gpu for mostly architectural viz rendering but cannot decide which way to go… I am considering between r9 390x 8gb and gtx 980ti 6gb or maybe you know any other good options? I am not sure what is more important - overall gpu speed or amount of vram. Is faster gtx980 has enought ram for most tasks or usually you need more than that and those additional 2gb would make significant difference . Or … 980ti with less ram but faster would be better because usually you dont need more than 6Gb ram? Also, is it currently possible to use amd cards normally for production without frequent crashing and bugs? I know amd is currently making alot of progress(at least from not doing anything to starting to work) but maybe nVidia’s card is still a safer option? Pluuuuus, do you think its worth it to pay additionall 300euros for 980ti?

Not all rendering featured are supported with CUDA or OpenCL rendering, see https://www.blender.org/manual/render/cycles/features.html

Cycles rendering is more mature with CUDA than OpenCL

Well, as Amd improving opencl support, i hope they’ ll add transparent shadows soon so maybe for architectural rendering this would be enough for some time? Also, what about vRam? Is 6Gb enough or its better to wait for something with 8gb then?

But what about amount of vRam? Is faster 6Gb better thant 8Gb?