Depends on how many you're looking to power.  The Bitmain PSU is very noisy but is quite efficient.  Per S7, it's not cheap, nor the EVGA G2 option.  You can see many people who own many S7 in this forum use the IBM 2K, 2880W, 2980W PSU and you get much better value per S7.  I'm using Dual IBM 2KW PSU to power 3 x S7 using j4bberwock 4K board.  Works like a champ and it's cheaper than $100 per S7 to power so you're saving a lot of money and when paired with the Ultra KAZE fans.  They're super quiet.  The foot print is much smaller than 3 x BitMain PSU or even EVGA.  I also own the EVGA G2 1300W and they're also very nice and quiet, but too pricey per S7 and you will need to buy extra cables or connect to only some of the ports.  If you have 240V, server PSU is the way to go. See how clean my cable management and how tiny those 2 PSU stacked on each other is.  My plan is to stack 4 of them on top of each other and be cooled by 4 x Ultra KAZE fans side ways.

http://i.imgur.com/h9eJbwK.jpg