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.
I only used a straw man because it is what I asked myself reading the post before saying anything else. If you want to discredit me because I used a straw man you could have at least kept that entire paragraph intact because my quote now looks as if I used a strawman lazily because I had no input.
I gave my reasoning to why the GTX 1060 will be easier to resell. I just think that the duration you plan to mine for is a factor to consider as well as electricity rates where you live or mine. I just think going up a series is better in most circumstances right now. The resale factor will become more obvious after a few years.
For RX470/480 vs GTX 1060 I am leaning to the RX series. The GTX 1060 does not scale with the GTX 1070 relative to performance for the dollar and this is more apparent on other coins besides ether. The main point of going for a GTX 1060 vs an RX card is because it does better on other scripts after Ether mining is over. To me it is RX470/480 vs GTX 1070 for now. Maybe I shouldn't have even bothered arguing for the GTX 1060 over the 970 in the first place.
What average are you reporting on your pool on a GTX1070, and what did you pay for your GTX1070?
I basically got an EVGA FTW edition (10+2 power phase) for 400 dollars after discounts and cash back. My pool average 24 hour hash rate bounces from 27-33. Most often hanging at 29-30. My miner reports a near flawless stream of 29.3 - 29.4