I'd go for the Panamera. I sold my '07 S 600 which I bought new.
Took a huge huge hit on depreciation. Posrche will depreciate less than the S classes.
I sold my 911 C4S cab, bought a Panamera and still had the S 600.
Started to become redundant having two sedans. the Panamera was more fun to drive and I preferred it to the S Class.
If you need the 3 seats in the back, and want the added luxury and quiet, then go for the S, but as a driver, the Panamera is more enjoyable.
Either way, both are great!