An old dilemma.
For 1440 rez the R9 390 aces the GTX970, but not by that much, at 1080 the situation is reversed. In a FEW cases, under extreme load the GTX970 can run into problems at 1440 rez when it starts to use the slow 500Mb of video memory, but the chances are you'll never see any issues, neither card is really fast enough to run 1440 rez with much AA enabled-and it's largely the AA which eats into the VRAM and causes the 'issue'.
Don't think the amount of VRAM is going to help with your video editing-plenty of system RAM and a good SSD will be of great help here, but not video card memory-but I could be incorrect so check your video editing software documentation. Also, older releases can be limited to Nvida CUDA or AMD Direct Compute GPU acceleration, and with some of that stuff being so expensive this may tip your decision one way or the other, because for gaming they're only a thick hair apart.
Recent changes to AMDs power saving features have helped reduce power consumption but Nvidia still rules here which means their cards demand less ventilation and can run off a slightly cheaper PSU.
My advice: Get whichever one suits your tastes or has the longest warranty, the 3.5Gb VRAM issue on the GTX970 is a massive non event and the difference in cost of powersupplies is also small: 550W for the GTX970, 650W for the R9 390 while the cooling issues on the R9 390 are is big as the VRAM issue is on the GTX970: microscopic.