My perception / experience between the brands.
390 :
vibrant / deeper colours / more striking contrasts;
smooth performance;
slightly faster than the GTX970 (less than 5%);
(in my experience) ATi / AMD have solid drivers;
more video memory for higher resolutions / vram hungry titles or mods;
generally run hotter;
consume more power (amounts to little in terms of £/$, though);
GTX970 :
generally lighter, paler colours - though still nice IQ imo;
even smoother performance;
slightly slower than the 390;
(in my experience) Nvidia have solid drivers;
less video memory, though a
vast amount of the time this won't factor when playing at 1080p - or even higher;
runs cooler;
consume less power (amounts to little in terms of £/$, though);
Normally, one could also say that the 390 would be better value for money than the GTX970, but it would seem that they're both around the same price atm, give or take.
TLDR :
As Whyscotty said, flip a coin.
