This thread is about a month old, but I would like to post my remarks as I tried both in my search for a MB but ended up owning a w124 instead (irrelevant I know).
The W140 is the last over engineered MB (dunno about the R129 and it is not included in the comparison anyway) and it only makes sense to my remarks as they state that the dimensions in both are close but over engineering led to having better quality and heavier and thicker materials and body parts (this means in both interior and exterior) for the W140 thus less space inside, bigger size from the outside and heavier overall weight. This means later not over engineered models are the opposite; more space, smaller size and less weight. I'm not saying that the W210 is of an inferior quality, I'm only saying that thicker quality parts typically have more quality than thinner quality parts, live longer and feel better (opinion plays a roll here, but I said that as a result of thinking objectively).
Now the real "it's a matter of opinion and taste" things start. To me, I prefer the W140 and I think it looks better and more aggressive. I like cars with more space, but the less space in the W140 is already big and more than enough for me. Truthfully, models after W140 and W124 do not give me the feeling of MB and feel somehow like any other car. I'd choose Lexus LS over post W140 S-Class.