The basic advice is to get the car with a more complete service history. That means everything done on time with documentation to prove it. But it seems that you have a fundamental question to answer - coupe or sedan? The SEC is enough car for 1 or 2. If you frequently have more people along and want to impress them, choose the SEL. The SEL will have a smoother ride because of the longer wheelbase but the SEC is not uncomfortable by any means.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD