The best thing about 1080ti is density. You got 1 card, 1 riser set, 2 power supply per 6xrig (or server psu) and you're getting 2 full 1070 rigs with cost of 1 MOBO, 1 CPU ,1 RAM , 1 HDD and 1 OS.
One 1080ti rig will never be 2 full 1070 rigs, especially if you are mining ETH.
Its ~36MH per 1080ti and ~30MH per 1070. So thats more or less like 1 1080ti rig = 1.2 1070 rig (36/30=1.2).
It actually depends on the scale of mining you want to do. 1070s will ROI faster if you just plan on mining at home.
no one should be mining eth...