Here is my take on the 3.5GB 970 vs 4GB 980 issue:
1. There are definitely games out there that runs VRAM right in that region, where 970 stutters where 980 doesn't, as far as I could tell, it does not matter if the game only uses that VRAM as reserve or has actual data on that VRAM, but the number of games that has VRAM usage lying right around that are few.
2. Anything below 3.5GB is fine with 970, it performs as well as can be expected from reviews
3. As you have said, VRAM requirements have jumped up in the past year, and I don't see any reason why it would slow down anytime soon, thus to me, it is unlikely that VRAM requirement is going to slowly crawl its way up, more like explode. Which means that if 970 can't cope now, 980 isn't very far behind. To me it makes more sense to go for a 970 now, save the money for an earlier upgrade if VRAM requirements does explode. It simply doesn't make much sense to spend that much more on a card that may not be much more 'future proof'.