What a terrible choice to have to make!

Seriously, bought a 1996 S500 Coupe (W140) in January... couldn't be happier with it. Amazing automobile.
These things are LOADED with options, so be sure to check them all and make sure you know what doesn't work before you put your money down. Many parts on the Coupe are unique and expensive. None of the body parts fit the sedan. Just for fun, call a dealer and price a headlight... :-)
Don't know if you've ever owned a V12 before... I used to have a Jag V12 and the insurance rates are outrageous. You might want to call your insurance company and check at the rate difference.
In my case, it was enough for me to give up my Jag... won't own another V12 due to the expense of insurance.

You didn't mention the years you were looking at, but I'd recommend a '96 or newer.

The cars are basically the same except for the engine. The S600 Coupe had a couple things standard in addition to what came standard on the 500 like a wood steering wheel, etc. Many of the 500s have most of the 600 options.

If you want more specifics on the car.
Hope this helps. KenP