Actually, both S55 and S600 have exact same suspension system.....ABC....only difference is wheels/tires that contribute to ride difference. You can put aftermarket wheels/tires on S600 if you like that are better than 55 config...be just as fast as S55....actually my 2001 S600 is .2 seconds quicker than current S55 (so say all car mags that tested)....don't know about speed specs between 03 55 and 600...but 600 is coming with 500hp+ twin supercharged V12....so there won't be much of a difference there. I've always opted for a V12....nothing is as impressive as the turbine smoothness of the engine!!!
Difference is that interior of 600 is like a Bentley...no plastic anywhere..even roof liner, dash, doors, seat backs, back trunk sill.....leather everywhere and much more burl walnut everywhere, plus a lot more standard options.
Go for the S600 :p