I have a colleague from work who was considering the S600 or the S550. He was told by the service advisor that MB bi-turbo V12 engines are much more exclusive and more likely to breakdown and harder/more expensive to repair. The 5.5L V8 from MB (like the previous 5.0L V8) is built like a rock and shouldn't have as many problems, as it is a high volume seller and should be easy/less expensive to fix, if any problems arise.

My colleague ended up getting a S550 4-matic, and he absolutely loves it.