SP20, SP35 and S4 are all NOTHING BURGERS .... I don't think they will ROI. Seriously, do the math and save your money.
Most of you have a mistake in your equations. They are based on wrong precondition - you somewhy think that BTC's exchange ratio will be the same as now. Try to imagine it will be 600$ in one month, when the miner will pay for itself in this case?
Trust in
BTC, otherwise everithing doesn't make sense...
Dumbass if you want to speculate on BTC price, do it right and BUY BTC.