I believe the 124 car to be more durable, and certainly the M103 engine in the 124 car is head and shoulders above the four cylinder.
There is much debate about whether the 123 or 124 car is the best all around MB ever built. Both of them are great cars. The 201 that you have is certainly not a bad car, but they were new to this size when they developed it. The subsequent models of this size were vastly improved IMHO.
The 124 cars have now gone done considerably in price in the last couple of years making them a good value assuming, of course, that you get one in good condition. There are plenty out there with reasonably low mileage and well cared for.
The Mercedes Benz Buyers Guide states something to the effect that the 300E is the best buy in a used Mercedes, period, end of story. Before making that statement, they do warn about the climate control problems that plague the 124 cars. That statement was made when the cheapest one you could find was over $10,000.