I drove a '96 W210 car about 75,000 miles after buying it new in April '96. I have driven a W124 car over 100,000 miles, and 123 cars about 700,000 miles.
The 124 is indeed a great and basically reliable car. Given that it is early eighties design and technology, it was far ahead of it's time. The 210 car on the other hand is nineties technology and design and is definitely a step up as far as driving experience goes. The 210 car is a much better combination of the solidity of a Benz but the driver experience and handling of a BMW.
If one costs about the same as the other, my choice would most definitely be a 210 car. If the 124 car can be had for significantly less money, I would have no problem choosing it for value.