If I understood it correctly from many sources, 1080 is about 50% more expensive than 1070 but only 30% faster.
I would aim for 1070 rather than 1080, unless you need or want it bad.
The jump from 770 to 1070 or 1080 is massive, somewhere near or even above 100% performance jump.
1070 is already around 64% faster than 970, which is already quite massive in my opinion.