Like so many things, it's complicated. What's your budget and what market are you in?
If you plan to play games on a single monitor at 1080p, 60Hz, a GTX 970 would be a great choice. For 2K , 144Hz, or more monitors the 390/X gets better. If you want 4K, or multiple 2K monitors or a 2K ultra-wide, then the 970 really isn't good enough, and I'd go straight to the GTX 980ti.
An 8Gb 390/X will do better at higher resolutions (2K better) and faster refresh, 120Hz or more.