That 380 is a fine engine - lazy, unstressed and long lived.

Change the rails, replace the chain and tensioner and fix any intake manifold leaks and you are set for another 100k miles @19mpg - the same mileage my wife's 1981 380SLC gets.

No speed demon, but quiet and long lived, and certainly more economical than the 560.

Others have said it well - the economic advantages are minimal. The lack of noise and smell in the gasoline version bears pointing out. I love my 300SD, but the 560SEL is a haven of peace and quiet by comparison.