Looking at the classified ads, I've found more "new engine, rebuilt engine, etc." on relatively low mileage 124 and 126 diesels with the aluminum heads. The W123's are all iron, which is a stronger combination, that lends to a longer lived engine. I also like the simplicity of the older cars. It makes a klutz like me able to work on 'em. I also like the classic lines of the older cars.

Ultimately, the overall reliability of either model depends on the care given to them. Inspect carefully.