Although I'll probably get incinerated for this, I will tell you that diesels are not better. They are different. They start differently, they sound differently, they have different torque characteristics, and the vehicle that they are in has different acceleration charactersistics.

Since you have already decided on a W123 coupe, then you must go and test drive both a 280CE and a 300CD. You will immediately see the difference between a gasoline engine and a diesel engine. Diesels definitely have the edge on long term durability and fuel economy, however, there are sacrifices to be made and they are not for everybody.