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.
Well, IMO if You are mining for ROI there is a lot of better altcoins (and algos for 1080ti) to mine than ETH that will bring You overall same or more BTC/ETH than 2x1070rigs.
That will kill ROI. Dont mine ETH with 1080ti. Mine alts and trade to crypto of your choice.
For sure autotrading is best ROI killer. When you are making ~3.5$/day but could have cashout once a week and get 7$ per card.
BUT if you want to hodl, yeah ETH could be good bet.
What do you mean by autotrading? Like the giant multipools that automatically convert what you mine to btc?