The GTX 1080 is significantly better than the GTX 1070, but, which GPU to choose depends on your monitor's resolution, refresh rate, budget, and power supply/connectors.
The GTX 1080 is ideal for 1080p/144Hz++, 1440p/~120Hz, or 4K/~60Hz AAA gaming in Ultra Detail. The Strix GTX 1080 requires 1x 6-pin + 1x 8-pin PCIE power.
The GTX 1070 is ideal 1080p/~120Hz or 1440p/~60Hz AAA gaming in Ultra Detail. The Strix GTX 1070 requires 1x 8-pin PCIE power only.