I can't speak from experience about owning the 124, however I have a 210 and I know friends that have the 124.
I've always loved the look of the 124 and to me that will always be the difinitive MB "look" but sometimes you just have to move with the times.
I have comparied the "touchy-feelly' parameters of the two cars. One of the most discussed areas is the "bank vault" feel of the door. I don't think that either car is any better or worst. They both feel great and expecially comparied to Japanese cars, they don't have the extra vibrations or rattles. I do agree with some of the other guys that the paint is better on the 124. I don't see or hear as many complaints about paint chip on the older paints compaired to the 'water based' paints. I do a lot of highway driving and have the rock chips to prove it. Can't imagine what it's going to be like after I lower the car.
From the seat of the pants comparison, the interior comfort of the 210 wins hands down. This is not to say the the 124 is bad, however if you had to spend an 8hr road trip, I would pick the 210.
So, I have the 210 and would not trade it in but if the right opportunity came up, I wouldn't turn down a 124 500E either