S7 is a good choice but you need to factor in energy cost and how much it is going to cost you to run and then work out Diff increases, value of bitcoin, amount spent on equipment and how long it going to take to brake even and then gain profit. Right now with the massive spikes it is hard to say how long it will take to RIO and even next year with the next half of bitcoin It gonna be really interesting to see who continues to mine and who switches off as am sure more will turn their rigs off and diff will come down people will turn rigs on again. But we are approaching no return and Bitcoin needs to put in place some type of system that allows transactions to be processed without miners as if all turns rigs off or many start sopping it is going to take longer and longer to process transactions and send payments.
I think your getting ahead of ourselfs a bit. I think there will be some big installs of Bitmain and Avalon (chances are some already done). Eventually bitfury will jump in.... possibly sp50 someday, and BW's miner someday to.
I think a awful lot of miners are in commercial settings where BTC will not slow down to much even if a lot of individuals hit power off. I wish I had a percentage of home/hobby miner vs commercial. But likely we will never have that. Just look at difficulty last period and chances are this period more gear is coming online and old gear is staying online longer.
A lot of us were mining profitable at 230.... so after jump it will take a decent amount of big jumps to shut us down. But I could be wrong. I just think slowing down to point it hurts BTC transactions will not happen unless some huge loss on value.