My experience is the following:
For ETH, the GTX 1060 3GB seems to be the best. I get 24MH/S for 75-80W (for around 200$) while with a 1070 I get 32MH/S for 90W (for around 400$).
Now if you want to mine ZEC for example, the result will be different, a 1080TI would probably be a better option.
You seems foolish buddy. GTX 1070 8GB card can be produce the 32 MH so you will be able to more income that card alone but if can get the electricity more cheaper in the sense we can make more money with that bro.
Performance wise alone 1080 8GB card is better than 1070. You can check in any site you will find the information states 1070i is the best card to mine.