Read the first two pages of this thread:
http://www.peachparts.com/shopforum/showthread.php?t=19772
Ignore the first few posts about the comparision between the W140 and W220 and review the posts about the differences from pre and post 1995 models that start around the middle of the first page.
Also, as everyone has stated, your best bet is working backwards through the year family tree: 99-98-97... I would consider first a low mileage-pristinely maintained 1997 over a doubtful mantained 1999, but besides that, equally mantained cars go for the newest one.