Up to about a month ago I would have recommended buying a 1070 Ti over a 1080 because the 1080 was still way overpriced from miner demand. For whatever reason, the 1070 Ti pretty much stayed under the radar of the miners. However, graphics card prices in general are falling and starting to return back to normal (at least on Nvidia's side).
I'm not sure what nation you live in, but in North America (US and Canada), the 1070 Ti is now only about 10% less expensive than a 1080. Case in point, EVGA's FTW2 variants. And if you shop around, you may find that a rebate offer on a 1080 even cuts that price margin difference even less:
EVGA 1070 Ti FTW2 for - $529.99 or $509.99 after $20.00 rebate...