With our price for energy here the SP31 will not even close reach a positive ROI, so my best bet would be the next generation SP50.

I'm really impressed by the Spondoolies-Tech company, they could have owned the market, but with such ridiculous long delivery times and only optimizing current technology it unfortunately is not leading anywhere. But I'm confident that (small scale or private) mining is not over yet, it's just a matter of putting more GH into one box and getting W/GH below 0.5 (off the wall).