Disaster Recovery
10 S3s, losing a unit only costs 10% of your overall hashing speed
1 SP30, losing a unit costs 100% of your overall hashing speed
Advantage: S3
This is wrong. SP30 is built using a modular design. The whole unit can't fail so that you loose 100% hashrate. Each chip is independently and also each PSU is independently. So you can fail either 1 or more chips or 1 or 2 PSUs (highly unlikely). Most frequent fails are in a chip or series of chips (SP10). One SP30 chip is around 5% of the hashrate so the advantage would be SP30 here.
I agree that the SP30 is modular and can certainly have single chips fail. So can the S3. I'm talking about failure. Disaster recovery. You lose that SP30, you're out 4.5TH/s. You lose the S3, you're out 450GH/s. It doesn't really have to be very dramatic. Rip the network cable out of the unit. Controller board fails. It can happen, and when it does, losing that SP30 costs you everything. Hence I gave the advantage to the S3.