GPU performance chart (average about 20 games):
https://www.techpowerup.com/reviews/Powercolor/R9_390_PCS_Plus/30.html
To me the choice between a GTX970 and R9-390 is a really tough choice. A year ago it would have been NVidia hands down, but AMD is improving their driver support.
Main differences:
1. NVidia has PhysX, Gameworks features
2. NVidia has MFAA (for supported titles) which is an optional anti-aliasing method which can increase frame rates by roughly 15% average but it varies a lot
3. NVidia has a lot more MONEY to work on drivers, virtual reality etc (Linux drivers are better on NVidia currently, and AMD may have a difficult time upgrading those. You may not care, but I added a drive so I could experiment with SteamOS for fun.)
4. NVidia's GTX970 uses less power (it's an issue for ME due to the temperature of my room at times. Electricity cost is minimal difference)
5. NVidia's RECORDING software is better IMO. (Shadowplay, though it may get renamed. I tried AMD's solution and didn't like it, though it should improve)
6. AMD has 8GB vs 4GB/3.5GB (mostly a future proofing issue. all things being equal it's a no-brainer but there are other issues. The GTX970 is just fine for every current game at proper settings)
7. AMD's FreeSync monitors are currently cheaper
(My research suggest NVidia's GSync has better potential going forward since they can add features into the newer GSync modules which the manufactures can then drop into existing monitor designs. And module price will drop a lot. In fact, with the module replacing the scaler, and NVidia taking some of the cost of researching future monitor implementations we might see price parity but with GSync being better.)
Having said all that, a good FreeSync monitor is a great experience. You need to get one with at least a 2.5X ratio of max to min asynchronous support. (if 30Hz is the minimum, then you need the maximum to be 75Hz or higher).
Anyway, those are currently out of your budget.
Summary:
Aside from the small cost difference (ignoring rebates), the main difference I see for you in favor of AMD is the extra VRAM. While it can and will benefit in future games the extend is uncertain. Game engines should also become better at swapping to system memory at appropriate times (which can reduce the VRAM requirement).
So personally I'd choose the GTX970 for the reasons I listed above.