Take a good look at this member's home page:
http://pages.prodigy.net/jforgione/MB_S500.html
It should clarify everything and then some more you need to know about a C140.
My personal opinion: the C124 is one of the best designed and proportioned coupes on the road, ever. I love the C140 too, although to mee it just looks a little bit thick on the C-pillar region from certain angles. Besides that, it is the King of all Coupes! Power and ride wise is miles and years ahead of a C124.
The green/parchment option sounds like a dream made in heaven (I really like that color combination, very popular in certain countries, not the U.S. though). The seat belt retractor is a common fault on C140s, expensive to fix if just exchaning parts for new ones, not that expensive if creative and send the faulty plastic parts to be made in copper as some owners have done here.
Certain aspects of the W140 reliability are sensible ($), although I do agree with JimF about being one of the most reliable, if not THE most reliable Benz I have ever owned.
The 2007 W221 is certainly a beauty. Reliable? Well, the model have been already around for a while in Germany and most worldwide markets and it appears to be better assembled than the W220, although I have already heard some horror stories about early 7-speed auto transmissions. Definitely wait two years on the matter and get back to discussion...
Is the W221 one of the largest S-classes ever made? Well, it is within just fractions of an inch in all directions compared with the W140. Hence both models should be two of the largerst S-classes ever made. The W221 just looks smaller compared to the slab-sided W140 (optical trick).