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...