Either simply overpowers at 1080p.
The 1070 can for example deliver fps in the mid 80s in Witcher 3 / the 1070 upper 90s (a pretty demanding title) ... at that point, do you care as long as you are maintaining 60+ fps ? As for cost, if ya wait a bit, when the 1080 Ti drops, you'll be able to buy that 1080 for very close to the cost of the current 1080's price.