In short, the GTX 1070 is aimed at those looking to max out games at 1440p and get into VR while the GTX 1080 is for VR enthusiasts and those looking to play games maxed at 4K and above. In my opinion, GTX 1080 would be ideal if you're looking to max out games at 1080p with a constant 144 FPS. Hope this helps!
EDIT: The GTX 1070 is for maxing out games at 1440p with a constant 60FPS, while the GTX 1080 will get a constant 60FPS on 4K. So, definitely go with the GTX 1080 for close to 144FPS if it's in your price bracket, but if you're okay with a constant 80-100FPS then go with the GTX 1070.