When you underclock TDP power for a 1070 it's much more efficient than a 1080ti. You can only get the same type of pull from a 1080 underclocking it to about 65% or overclocking the power to 100%+

1070's will pull much less power for more watts. I haven't tested all algorithms, but for the 3 most profitable - 1070 def worth it. Buy your gpus one at a time so you can dca them as you go. Remember if you buy them all in one go you have to set it all up asap. I'd say buy one at a time and find the sweet spot for overclocks as you go!