If you can afford the r9 390 and your PSU can handle it, I will still recommend it, since it becomes quite better with dx12 (more futureproof), if you can't afford it, then the gtx 970 is a really strong card in its own right.
Single gtx 970 is definitely better, single more powerful gpu is always better. If you plan on going for more than 1080p then you might even consider the r9 390.
you will be better off with one single 970, most games have some degredation when you run cards in sli such as not being able to run anti aliasing as far as it could go and other weird things with the graphics settings it is better to just avoid those issues and go for one card.
If you already have a 960 4gb, then SLI will give you more performance than a single 970. But if you are buying new now, then its always better to get the single faster card.
Single gtx 970 is definitely better, single more powerful gpu is always better. If you plan on going for more than 1080p then you might even consider the r9 390.
If you can afford the r9 390 and your PSU can handle it, I will still recommend it, since it becomes quite better with dx12 (more futureproof), if you can't afford it, then the gtx 970 is a really strong card in its own right.