If you want to do a little search on facts about pre and post facelift W140s, there is some useful info at this thread:
http://www.peachparts.com/shopforum/showthread.php?t=19772
Just to clarify your questions:
Pre facelift models (1991-1994) had a better standard leather than postfacelift models (1995-1999). On the 1995 post facelift models M.B. added a second option for leather seats (at least in European and worldwide markets) called "Alcantara" which was better than the original prefacelift leather (this leather could be ordered in a one or two tone variation).
To save production costs many chassis and electronic items were simplified at the 1995 post facelift models. Examples: the doors had 45% fewer parts, the A/C controls were simplified and costed about half of the original unit, etc...
Some of the reliability issues of pre facelift models were corrected on post facelift models, although not all:
The A/C evaporator problem persisted throught the ENTIRE life of the W140, there is no distinction on that.
If you would like to buy the best pre facelift model: 1994 (although, be prepared to fork big $$$ for proper maintenance!)
The best pos tfacelift models: 1998 and 1999.