Both are extremely powerful cards.
8gb 1080 has GDDR5X while the 1070 has GDDR5. Meaning the 1080 has faster memory.
1080 is designed with the future in mind in terms of optimizing for VR and stuff like that.
Both are extraordinary cards and can play just about any game at ultra at the moment.
I have a GTX 980ti which is less powerful than both those cards and I manage to play almost every game 4k at High or Ultra, so the 1070 should do just as good if not better.
You can't go wrong with either, if you wanna save a couple hundred just go with the 1070.