dont know what site you use to determine the difference in performance between gpus. passmark has the 1060 performing 50% better than the 960.
if youre looking to future proof a card for more than a few years, you need to look for 6GB+. Tech seems to move ahead pretty swiftly, but games companies want to sell to as many people as possible, not limit the sales by demanding 1080 minimum spec. Most games optimised to take advantage of 'high end' 4GB cards; even if the cards arent high end, 4GB is the current 'standard' in high end. It will move to 6, then 8 etc every 5+ years. Im not saying this is true to life, but remember when the 880 came out and people were saying 8GB vram was overkill .... enough said.
imo its better to buy cheap and often, rather than expensive and never (though it depends 'how' expensive you go eg TI SC or a titan). buy a 1060 6GB and then with the $300 you saved, next gen buy a 1160? which will most likely outperform a stock 1080.
wheres those graphics card experts !