The 560 will burn more fuel. Either engine is good if maintained properly. Check cam chains and tensioners.

560 also has hydro-pnuematic rear suspension, can be $$ to fix when it fails.

I have a 420 SEL with just over 200K on it and no major problems, but both these cars usually need a little front end work at this age. Have a mechanic check them out, both are fabulous cars, the best MB ever made (W126)