Broadly speaking the fanboy nonsense is just that.
The real issues here are Price, Performance & Driver support.
AMD is usually cheaper
At 1080p depending which site you look at either the 970 or the 390 will be marginally ahead depending on the games chosen. By all accounts the 390 performs better at higher resolutions but there is no single card solution (as yet) to 4K gaming. The (fast) 8GB or VRAM does give the 390 an advantage at higher resolutions and with certain modded games.
Driver support belongs to Nvidia, AMD suck at it, and their innovations, while often very good don't count for squat if developers don't code for them. This is where Nvidia shine, they supports developers well and often pay to have them support their innovations. With AMD you have to know what you're doing, as it's often up to you to fix what AMD can't in any given game.
All that said, I bought a 390 as I play Skyrim and I wanted 8GB or RAM for cheap. I've seen video of a 4GB Nvidia Titan stutter in modded Skyrim as it only has 3.5GB of usable VRAM, and a modded game can eat that for breakfast, which allied with the game's engine behaviour can cause problems. The 970 is now an old card, it may not fare well in the coming years, depending on how often you change video cards. I do it every 3 years or so.
I have Witcher3 and hair works aside, (pure Nvidia, and GPU heavy) I get 60fps at Ultra, Everything else I've tried, is smooth.
If you're uncomfortable with digging into your PC's guts, and fixing problems when they occur, or your replace your card every year, then buy the 970.
YMMV