What zyh1987 said is a good reason to buy a 1070, but let's also put it in this situation:
In a few years when 1070's and 1080's are getting more un-efficient for new games, when a 1080 achieves 60 fps in a title, the 1070 will be 25%ish less, meaning it will be in the 40's (Playable, but not smooth and pleasing to the eye)
Also, if you are playing on a 144hz 1080p monitor, it would be a better option because in some games it doesn't even achieve it (On maxed settings) for example;
Far cry 5 1080p maxed out runs around 90-110fps on my Gaming X, keep in mind this is with an 8600k (One of the best gaming cpu's)
GTA V 1080 maxed out runs about the same as Far Cry. And then you have to factor in the 25% less performance with the 1070.
If I were you, I would look up benchmarks of the specific games you are looking to play and go from there.