S5 is the best deal at the moment.
@142 USD it will become unprofitable.
S4 is quite risky, especially if you watch the market closely. There is very less trades there.
S7 will be a good buy @ 0.0012 0.0013
The fact is that with current prices of the s7 you will never return on investment, nor with the s5, when you count the rise in difficulty until the btc halving. the only chance to ROI is with btc price raising. But then its making any sense to buy hash or a miner, just buy btc and hold them.
here is my basic calculation:
1000gh:
0.007 * 0,7=0.0049 current difficulty earnings
0.0045*0,35=0.0016 difficulty*1,5 earnings before halving
0.0049+0.0016= 0.0033*230 = linear raising and at the end of 7 months you will earn 0.76 btc, but you paid 1.2 btc.
without btc rising your miner is obsolete after the halving, new generation machines took over btc mining or btc will rise, but then its more profitable to buy and hold btc, so you make a gain, otherwise you just cover your loss.
I am not a prophet (i wish i could) but saying that right now the hardware price could be overpriced by almost 40%.