IIRC, the W140 is known for particular problems including front end shimmy and AC evaporator failure--the latter being one of MB's ridiculously expensive replacement jobs due to the need to dismantle a lot of car in the process. The E-class, both 124 and following, have fewer problems related to body and systems issues as I get the impression from the BBs. The engine situation of course is what it is for a given motor in either body. Having said that, the 140 seems to be one incredible automobile when it's properly sorted out. I'd do some significant site searching on both models.
'97 SL500, 40th anniversary edition
'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)
SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)