It really depends on what plans you have.
The GTX970 is a great card for the money nVidia is asking for it, even with that "detail" that has the last 512MB of it's 4GB is slower. There is hardly a game that pushes above 3GB unless you go multi monitors or 4K.
Now, if you don't care about power consumption, think of going into multi monitor set ups AND plan on going with more than one card eventually, then the 390X is not a bad idea either. The 390X'es 8GB will come in hand at greater/bigger resolutions, but in terms of performance at 1080p, the 970 is the better all-around card in my opinion. Even better than the 980 at 1080p from a price/performance point of view.
Cheers!