I expect Nvidia considered 8GB for the Titan X but that would have meant a memory config like the 970 3.5GB + 0.5GB, no doubt Nvidia wanted to avoid a lynching..
6GB / 12GB works perfect on GM200 without compromising the chip, but they couldn't of justified only 6GB as well as nerfed compute on the Titan X (The reason for OG Titan price).
So now we have 12GB which let's face it is just massive overkill but goes some way to justify it's high price tag. By the time we
need 12GB Titan X will be lacking in GPU grunt compared to newer cards.
It will be interesting to see how many people claim 12GB is needed but then go onto praise the 980 Ti with only '6GB'. It's a fickle thing the graphics card forum
8GB will be absolutely fine for gaming for a long time, and by the time it's not those type of buyers will have upgraded anyway
+1 cost is what will motivate me to make the switch beyond anything else, if performance is on par but costs much less I will sell up and go Radeon. If performance is a lil better but very expensive I may just stay put with my TX for now. Depends on how much I could get back on my card.
The Titan X is a beast, no probs with the card at all. I just don't want it's value to be lost, so keeping my eye on 390X release to see if I need to sell up..