Actually GameWorks is a hated entity by most of the devs in this world... barring the guy who made Project Cars..
The fact is that nVidia endorsement is a thing devs want to make their game a success and reduce costs.
However DX12 is, without question, the future of current Windows gaming as it will also share XBox One fundamentals.
Thus not at any point would a GTX 970 be recommended over an R9 390 because it's simply outclassed.
You mentioned some games are leaps and bounds ahead with the GTX 970 vs. R9 390.. then let me ask you which games those are and how relevant they are.
WoW, as big as it is for an MMO, is f.ex. not a relevant entity for a card in that performance bracket..
Following up on that as AMD is pushing, and very much succeeding, in Open Source progress and it's GPUOpen project nVidia is running scared.
This due to the simple fact that any developer whom wants to succeed and progress with all the bad rep nVidia has gotten with GameWorks is very much considering abandoning it all together for something that won't require selling your soul to the devil.
As a proof of nVidia running scared they made the culprit which they used as a weapon (and some others) Open Source on GitHub.
NVIDIA Releases HairWorks, Volumetric Lightning and FaceWorks Source Code on GitHub – HBAO+ To Be Added Soon
So if you combine the crap that GameWorks represented in ruining not just AMD's performance but their previous generation of cards as well along with the fact that nVidia blackboxed the whole thing with closed source vs. a system that performs inherently better with the future go-to API along with full open source disclosure AND not the inherent rep that came with the GameWorks titles that screwed so many over ... are you really surprised?
No-one says the GTX 970 is a terrible card, they are saying with the options available right now it's a worse choice than the R9 390.
And before it's asked why:
In relevant high end games they perform equally to better than the GTX 970 on a 1080p resolution.
In older games the FPS of both are so high that it reaches 3 digits making it useless to judge them apart as you won't have a monitor that can keep up.
In higher resolution high end games the 970 is thrashed by the R9 390, the 970 simply doesn't stand a chance.
Yes in
SOME games the 970 can run some things smoother due to GameWorks being a dick to AMD and older Kepler based cards.
However to gain that bonus in a low amount of games you sacrifice DX12 performance enhancement capabilities which are present on the card for free.
And before you state "Well DX12 isn't here yet!" or "We don't know what DX12 will bring!" ... well you'd be wrong on both accounts.
Microsoft themselves have stated that DX12 adoption rate is so much faster than any DX adoption before it.
The DX12 API is also built and brought out... it doesn't mean there may be features in DX12 that are as of yet unknown.. it means everything is already known.
AMD, Intel, Matrox and nVidia (the only players left in the market) have everything that is DX12.. they are supposed to build around that API not the other way around, there will not be any new and mysterious offerings that are unknown that no-one has discovered...
DX12 is here and it's bringing the hammer down on DX11 and to add to that:
The features of DX12 as a basic premise which BOTH AMD and nVidia have to adhere to to get even a resemblance of "OK" from Microsoft is the fact that Drivers need to play less of a factor to the point of almost needing to be eliminated to be any hindrance to performance in games and it is left up to the developer of the game to dictate optimizations so no "Driver update will fix game performance" in a properly developed DX12 game.
Unless you are brand loyal (note: I did NOT say or mean fanboy because they should have 0 place anywhere in the techworld) or REALLY SPECIFICALLY MUST HAVE THAT LOWER POWER DRAW there is absolutely no reason to pick the GTX 970 over the R9 390, especially not with the progress the Crimson drivers are making.
That said .. my current recommendation is wait till about July / September and make your choice then.