It's hard to compare a 16 year old E Class Coupe over a 6 year old C Class, becays they are tottaly 2 different cars made in different engineering specs, you are right the C class is newer its the car that passed the millenium, but if the coupe is in good shape and perhaps a low KM car I would go for the Coupe because it will outlast the C class any day, you know why?? Because it's a W124!
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In the world of roads, there are those who travel and those who own it. The choice is yours.
I wouldn't trade a clasy sporty coupe for a newer small sedan. The coupe will hold its value a lot better and it is not made as good as the 124. Keep the coupe.