In an ideal world and if we know nothing about the W124 and W210 5 and 10 years after they are new, I would say it is a toss up. I am still not quite used to the round eyes of the W210. I prefer the look of the W124. Slender like a big cat. The W210 is fat like a pig.
But the W124 has proven to be a long performer since 1986 and the W210 has not yet . So based on that, I would say W124. BTW, the dash on my W210 has several cracks on the stresss points. I do not know why MB puts the cheapo vinyl on it.