S5+ costs ~2300$ new and hashrate 7.800 (@bitmain/"up coming)
S7 costs ~1800$ new and hashrate 4.800 (@bitmain)
For me s5+ looks better.. more hashrate and only little difference?
Noise is not a problem.
Where are you seeing S5+ for 2300$? They were sold once in very very limited quantity and then never again. It was just a design prototype for using up chips and a proof of concept for the S7 cooling designs.
Your options for buying in bulks are S4, S5, S7 and Avalon 6. If you find a private user that bought a tons of S5+ then and is now liquidating, i guess you could find a decent amount of S5+ but thats it. But S5+ for 2300$ new is a horrible price either way.
And a S7 costs under 1000$ new, closer to 900$ per. I am not sure why you are saying 1800$ per S7. You're looking at a 100% markup from the real price here. Double check your prices before getting ripped off royally.
TLDR at the price you "found", in truth the S7 brand new in bulk is way, way cheaper. But in actually accessible GH/$, its going to be S5's in bulk or S4. For example there is a guy selling like 39 S4 on the marketplace.