I drove an S550 and a 750 back to back and for me there was no comparison, the 7 was just the better car. I liked the S from an outside look point of view but that's where it ended. The interior quality of some of the materials was just sub par for a car of this class (e.g. the temp controls in the S class are cheap looking and when you pushed one up the one next to it would move a little, the COMMAND control knob with it's ridiculous little keypad cover thingy, etc...). The seats in the S were nice but I liked the once of the 7 series with the individual leather better. Also, the 7 series twin turbo charged engine just had a lot more of the 'kick-in-the-pants feeling' to offer than the naturally aspired V8 of the S. The low end torque of the 7 series V8 engine is just AWESOME.
Anyway, that's just my opinion and everyone has different points of view and feelings about this subject