Depends on how you're going to run the car : the W126 was the successor to the original S Class ( the W116 ) - both of these models borrow heavily from the legendary W123 : designed by engineers who had the first , middle and last words . These cars were heavily over engineered and are mechanically very robust - due to the ages of these cars , rust can now be an issue , but many are in surprisingly good condition , thanks to the superlative build quality - Mercedes even stated prior to release that they (the W126 series ) were designed to last into the new century - so a lifespan of 20 years or more was always part of the plan ( how many current cars could you say that for ? ) .
A W126 can still be very well equipped , with features such as alloy wheels , central locking , electric windows , sunroof , power seats , illuminated visors , air con being by no means unusual ; and other options such as SLS , fridge , electric blinds/curtains in the rear , individual rear seats and trip computer being less commonly found options .
The later W140 series is still a very well engineered car and certainly has a lot more electronics in it , as well as a longer list of optional extras : features like double glazing , door closure systems ( which can be tempreamental ) more sophisticated heating and ventilation systems .
However to compare a W140 to a W126 is rather like comparing a W124 to a W123 . Certain things have been 'pared down' to save on cost and weight , so the later cars are not quite so mechanically robust as the earlier ones , although they are still very good .
I think I would sum it up by saying - " If you like to get the spanners out and look after your cars yourself : get a W126 ; on the other hand , if you do your maintenance with a chequebook : get a W140 " .
The German S Class club website ( with pages in English ) is a useful resource with an excellent buying guide
Mercedes-Benz S-Klasse Club e.V.
Other useful links include the Dutch site
Mercedes S-Klasse Club Nederland - voor alle eigenaren van een Mercedes W116, W126, R129, W140. | Mercedes S-Klasse Club Nederland
W126Club Forumoverzicht
and this one
W126
A useful contact for W140 owners
Mercedes Benz W140 S Class Spares and Parts
Sadly , not so many other sites out there dedicated to this model , although the official club does have a section on its forum dedicated to this model .
The Mercedes-Benz Club