Using more stable coins since it'll take about 9 months to ROI a mining rig.
7 x 1080 Ti $700 per card
ZEC 4700 H/s @ 1400 watts = $19.98 per day
7 x GTX 1070 $450 per card
ETH 224 MH/s @ 770 watts = $13.38 per day
Value of 1080 Ti
19.98 - 13.38 = 6.60
6.60/13.38 = 49.3% more revenue
Cost of 1070 $450 * 49.3% = $671.85
With 1080 Ti you're paying an extra $28.15 per card.
With GTX 1070 you'll have to build more rigs; paying for motherboard, cpu, riser, etc.
Basically the same, both 1070 and 1080 Ti are equally as good. I prefer 1080 Ti since there are less rigs to manage.
Another thing to consider is if you would rather use the money to buy the coins.
BTC is up 5.4x and ETH is up 23.9x within a year. Would have made more from holding coins than mining.