If you wanted to find out how GPU's stack up, you can always search up 'benchmarks'. These give performance statistics in comparison to other graphics cards. If you were considering buying this PC with the 390, and then buying a 970 to replace the 390, I would definitely advise AGAINST it. The performance is incredibly similar, there is no point to waste money. At higher resolutions (1080p>) the 390 will beat the 970 slightly, (Maybe 5+ FPS), but the heat, power consumption are much greater with the 390.
In short, if you have to decide between the cards, (You have both or have neither) It comes down to personal preference. If you have one but not the other, there is no point in paying for the other.
Any questions, feel free to reply, but I will only be online for the next half hour, then offline for maybe 6.