W220 purchase over the next few months. S320CDI or S400 CDI?

Hello. It’s about og. models. With autogas they are still too expensive and have had 3 cars with gas..all only had worse. so it should be nn diesel in the next few months I think. Which can you recommend? What should you be aware of? Vulnerabilities? ROST??? Repairs??? VIelen Thanks.gruss, daniel