NVIDIA is butt buddies with a bunch of gaming companies, so they get their updates out first, and AMD has to follow up. The R9 390 will give you better/comparable performance in games with a far larger block of VRAM.
AKA: later on, if you want to upgrade your PC but not break your bank, wait a year or two, get a second R9 390 for a cheaper price, and CF them together. You'll have way more VRAM than an SLI'd 970 setup and at that point, your VRAM might be your blocking point.
NVIDIA is better in games pretty much only due to their drivers and updates. Otherwise, AMD is comparable if not better, although with much more inefficient chips with higher TDP/thermal outputs.