I was toying with the idea of getting a freesync monitor maybe even a 27" 1440p one, that was partly the reason for wanting a new GPU (although i'm not sure whether a 970 or a 390 will be able to run these titles on ultra at that res). It seems to be a pretty even debate, although I might have swayed into selling my 380 to go for a single card if i'm honest, just seems like less hassle.
First of all the 380 is the old 285. So to play on ultra everything needs better card. (and we do not know your CPU).
After that 970 or 390. I do not understand how you even think about it.
Clearly 390.
Is faster, not having issues and most important for the same money you buy the 8GB version not a crappy 3.5GB card which theoretical has 4GB.
But for all on 2560x1440 Ultra you need to be thinking for Fury at least 390X or GTX980. And Freesync/Gsync
So my advice to you, if you are looking there, get the Freesync monitor first.
You can use it with the 380
today and then you assess the situation of how much grunt power you need based on your experience.
As a rule of thumb. A 290X is good enough to run 2560x1440 on pretty high settings if not ultra. But all depends the games also.
eg Total War Attila the engine is crappy at best and even I cannot run it at Ultra with the 295X2. While other games shine with single GPU on their own.
If you have 1K to spend right now, just buy the lot. Otherwise, just get the monitor first.
Thats my advice.
