https://www.youtube.com/watch?v=RzBMkrAbHlQ
https://www.youtube.com/watch?v=QhA2eCOEjHo
https://www.youtube.com/watch?v=lYQT8udtuWM
https://www.youtube.com/watch?v=D0YOdL4A0lg
https://www.youtube.com/watch?v=gkmLf0WjGjo
https://www.youtube.com/watch?v=mHlN8s7ENuo
They are actually pretty close in performance with the 390x pulling ahead of the 980 in some games.
the 390x is cheaper and has twice as much vram and a huge memory bus which is nice for high res textures and future games.
the gtx 980 consumes way less power and is less hot, it probably overclocks way better too.
Ive also heard that will release some drivers that will improve performance even further.
so i did some math and if you ran both cards on max load for 1 year in an area that costs about 15cents per kilowatt hour, the r9 390 would cost $100 - $200 more in power (depending on which benchmark you take numbers from) than the gtx 980. Obviously this is an unrealistic scenario but it is good to know. Check how much electricity is in your region.
The two cards would probably end up costing about the same if you take power for a few years into account, you would have to do quite a bit of gaming though. Like 4 hours a day for 3 years with 15 cent per kilowatt hour with the highest benchmark numbers.
they use about the same power idle depending on which benchmark you look at.
I would personally go with the 390x as games are increasing in texture resolution and the 390 is better in 4k, and amd will release new drivers.
P.S. my current card is a gtx 970 so i am not an amd fanboy.