My rig of 6x RX480s with optimized power runs under 700W. That's 140MH/s+ if I'm conservative.

I'll reach ROI before you.

Building another 4x rigs as soon as the rest of the RX480s and risers arrive.

I've looked at the GTX1070 thoroughly before settling on the RX480.

But yes I do agree that we need a fallback plan in case Eth doesn't work out. (But it should)

In either case the RX480 just has far better compute capability per watt.
The amount of the Compute Units(AMD)/ShaderMultiprocessors(Nvidia) are just superior on the RX480 compared to the GTX1070.

36x CUs on the RX480 compared to 15x SMs on the GTX1070

4x SEs (Shader Engines) RX480
3x GPCs (Graphics Processing Clusters) GTX1070
Verdict = same

9x CU (Compute Units) per Shader Engine RX480
5x TPCs (Texture Processing Cluster) per GPC GTX1070 (Each TPC Contains 1x SM)   
Verdict = RX480 wins




My power is free as I can use as much as I need at the moment. Even if that means 10kWh. Included in your rent usually means a small amount for household use.