DX 12 does not look promising for NVidia at the moment. The question of whether they can get their Asynchronous Shaders queue to work in conjunction with the rendering queue has yet to play out. This will favor AMD in DX 12 titles. However, until such time as there are DX 12 titles to play, go with what you see producing better results for what you already know you can run on the card.
If I had to choose, I would go with the Fury X. Frankly, both the 290x, 390x appear to have more raw horsepower than a Titan when it comes to raw draw calls. The problem has always been in the implementation of those resources in software. DX 12 stands to alleviate a lot of the hold ups there. A Fury X clearly has the goods to walk away from the 390x class of cards and crush NVidia if things work out well in the software department, but it's going to be up to game engines to utilize the power that is available to them. 2016 stands to be an interesting year for game releases.
I wouldn't touch the Fury X because it requires a screen with a Display port connector if you want more than 4k@30. The unfortunate decision to not equip any of the current gen AMD cards with an HDMI 2.0 means you're pretty much dismissing a full line of affordable 4k screens that only support 4k@60 over HDMI. If AMD updated the HDMI out on their cards, I would consider them. Until such time, I can wait.
I personally don't see NVidia's 980 Ti as worth the money, knowing it's likely never to have improved performance over what we see now, and as it stands, under DX 12 Asynchronous Shading situations, falls apart.
Edit: The only vendor specific code in the DX 12 benchmark numbers is from NVidia. While the game may have ties to AMD, the bad performance is 100% due to how NVidia has handled the asynchronous shaders in their drivers. They told the developer to specifically disable the feature for all cards, but the developer said no, so the vendor specific code only disables it for NVidia, so their numbers don't look quite as bad as they were under DX 12.