It might simply be a matter of future proofing. Both are great now, but the 1080 will last you longer so you won't have to upgrade as soon. Whether that's worth the price difference is up to you.
I just bought the 1080. They were close enough in price, and I wanted the long-term option so the difference was pretty minimal to me. But if you're on a budget, the 1070 Ti will do fine.