I have a question about either of these two cards (the 390X and the Titan X). Will they run Star Citizen at 4k Ultra settings and still get at least 60FPS average?
Well according to some people the 390x will be ushering in a new era in power and performance, simply being in the same country as someone running one will allow you interact with god himself.

./sarcasm off
No, I very much doubt a single card of either will be able to run newer titles at 4k ultra at a consistant 60fps. Maybe if your willing to sacrifice on some of the really 'flashy' settings and have a gsync/freesync monitor it will be OK.
That being said it really is hard to be accurate with the 390x, but my gut feeling is while it will be a top card the tech still isn't quite there, although possibly a lot closer when AMD's new card actually arrives.