GTX 970 if you play at 1080p. It's still faster than any AMD card at the same price bracket. R9 390 if at or higher than 1440p and you don't mind the power consumption.
Most cards are limited by shaders rather than memory. That's why it doesn't really matter if you get a 2 or 4GB GTX 960, or a 8GB R9 390 instead of a 4GB R9 290. The difference will be minimal. It will likely border 10% tops at 4k and that's it...







