Well. Depends how long you plan to mine for. With that said, you will have a much easier time selling a GTX 1060 in a few years than you would a GTX 970.
Straw man argument - Ethereum mining isn't going to last a few years.
Also, the price of the 970 has ALREADY dropped quite a bit, and will probably be fairly stable for a while now, it won't be hard to sell as it's STILL a quite powerfull card.
Mostly, it comes down to the 970 being LOWER COST than the 1060 vs more power usage as to which you want to get.
The real argument though if you insist on efficient cards is the RX 470/480 vs the GTX 1060.