OpenCL/OpenGL is much more widely adopted as apposed to CUDA. Strictly because it's proprietary and exclusive to Nvidia. and will ONLY work with Nvidia products. OpenCL/OpenGL works with everything, Intel, AMD, Apple, Linux, Windows and is very portable. mind you there's no guarantee that your code will work, effort must be put on your part to make something work. but to keep it short, OpenCL/OpenGL is much more open, CUDA isn't. also, Nvidia cards aren't good with OpenCL/OpenGL like AMD cards are. AMD has heavily invested onto OpenCL/OpenGL. AMD is very generous when it comes to alot of things. almost to the point where you can argue they are most likely losing money giving you the stuff they give you with their GPUs.
PhsyX is realistically a joke, if you want to get down to it. it hasn't been widely adopted. and is only available on Nvidia titles, it's not available anywhere else. and even some Nvidia titles today don't even have PhsyX on it. strictly because some developers have gotten better at making physics in game that they didn't need to use a Proprietary physics made by Nvidia that only works on their cards only. oh and i forgot to mention Nvidia has changed PhsyX quite a bit, most of the PhysX stuff is now implemented on "Gameworks" and all that stuff is CPU based as apposed to GPU.
If you have an AMD card however PhsyX will be run on your CPU but it will put a lot of taxing on the CPU. which as of now won't make sense anymore considering, DirectX 12 and Vulkan (OpenGL 4.4) are starting to put ALL the workload on the GPU. so you may not need a fast CPU anymore.
As for the 970 versus the 390.
they are neck and neck. almost identical in performance. BUT the main issue everyone tends to argue is that, the 970 has a VRAM issue. they all do. When you purchase a 970 it clearly states on the box 4GiB of VRAM on the box. that is true for the most part. BUT, 3.5GiBs of the VRAM has fast memory. the other 500mbs is slow as hell. so once you go above 3.5GiB of VRAM, the GPU will slow itself down. which let's be realistic, give it about a year, and games will start going above that. so to keep it short the 970 isn't good for the long run.
I do recommend you grab the 390. it's the overall better choice in the long run. PLUS you don't have to buy into Nvidia's proprietary non-sense. like G-Sync, Game-Stream, and Gameworks, (Which is bad for gaming as a whole) all Gameworks based titles have taken backlash for destroying everything that isn't a Maxwell based GPU. there has been people complaining how a 780ti was being beaten by 960. which is not supposed to happen at ALL.
Here's a good source on OpenGL/OpenCL vs CUDA