Peter,
The S500, S600 embodied in the W140C chassis was the best car ever made. I'm a 'little' prejudiced
or maybe a lot!
The '94, '95 or '96 were some machines. I just bought the '94 S500 cpe about 6 months ago. I love it. It rides, and runs beatifully. The gas mileage is a bit tough, 15 mpg city, 19+ mph highway. But with today's high gas prices, that is probably a big minus.
Click this link if you haven't seen it, to compare the two cars (W140 and W220).
http://www.peachparts.com/s500.htm
Maybe this will help you decide.