Actually DOHC V12 is in fact 2 M104 "merged" together. In-line-six design is know for its smoothness nature, and V12 makes V8 feels like Small-Blocks on old muscle cars(vibration, noise...etc). And with seperate ignition for both banks, the V12 would even run on one bank of cylinders in an unthink-able event.
If you are looking for a W140 cruiser, S320 is more than enough. V8 models give the best compromise of performance and mileage. V12 redefines ultra luxury sedans...