If you're using a 60Hz 1440 or 4K monitor either option is good, in both cases it's more likely the GTX1070 will be the hardware holdup, in less demanding games the system will, obviously be held back to 60FPS regardless of configuration.
If you're on a fast 1440 display the vote would be split but I'd favour an i7 7700k.
For a fast 1080 display the i7 7700k rules, no question.
Also bear in mind your overall uses: For most users the i7 will be more than enough but if you regularly use heavily threaded serious applications like rendering, photo editing, video editing or 3D modelling software a R7 1700X chip will virtually eat an i7 7700k for breakfast, it'll also show a clean pair of heels to the Intel part if you stream or record during gameplay.
If you can wait, Coffee Lake SHOULD be officially launched early in October, it may be wise to wait until the reviews are in before making such an expensive decision.