r9 390 is better all around.
the gtx 970 has less than half the usable vram at 3.5gb
when it uses its last 3.6-4.0gb, the card will have performance issues.
even in the same scenario at below 3.5gb, the r9 390 still will give you a few more frames.
vram is most important at higher resolutions, but those cards are optimal at 1080p. 1440p needs a lot of power. im running 1440p with a 980 ti around 40-50fps ultra with aaa titles. expect to dip below 30 with the same settings on a 970/390.