I just started S3 mining and went though similar PSU choice issues, in the end i went with 1:1      1 psu per 1 S3.

To give me some flexibility I went with a FSP Raider 750 Watt 80 plus Silver PSU, it has 4 6/8 pin pcie connectors so I can fill all the S3 power connectors,
apparently this allows you to overclock and keeps the unit slightly cooler per given freq.

Secondly, it has a common 12 V rail giving 60 amps which is more than enough for an S3 and allows the PSU to run at around 50% capacity which has an efficiency over 90%.

One thing I have noticed with some Gold modular psu's is they have multiple 12 V rails and may add up to 700-800 watts but only deliver 18 amps (per rail),
 if you power you S3 with one connector per side from a rail like this you are going to have trouble as an S3 needs 24-30 amps depending on who you read.

With lots of PSU's it does chew up more space and power points but you don't have a single point of failure and your not running your PSU at 90-100 % capacity.

Good luck with your PSU hunting.