Go with Benz
I owned a 89 Benz 300E, had it up to 260K miles, no problem whatsoever, except wear and tear. At the end, the head gaskets leaked & engine loose compression, and dealer wanted $2k to fix it. So I offload it to my buddy. He spent less than $200 to buy the parted, and fixed it himself. The car has more than 300K miles now, and still run smooth & powerful like a charm.
After the Benz, I bought a 98 BMW 528 and a Lexus GS300. Both had 50K mile when purchased.
The Bimmer had so many problem: anything that could go wrong did: water pump, oxygen sensor, sunroof, ECU, radio, climate control LCD monitor (very comon problem for Bimmer, and very expensive to replace), window switches, cup holder (replace 6 times !), cat converter (cost me a small fortune). I think except for the transmission, everything else in the car were fixed one time or another. The car is fun to drive (when it runs, which is ...rare :-). After two years of nightmare, and when the car failed smoke check, I traded in the car for a 99 SL500 instead of spending another $3K to fix it as dealer suggested (it had 90K miles when traded in).
The Lexus never had a single problem. Beside oil, brake, tire, timming belt, all it needed were maintenaince every 60K mile mile. It now has 220K mile, and still look like new, and run very powerful & smooth (even my mechanic is surprised by its condition).
I also bought a 99 Benz 320E lately. 4 months of ownership so far, and no problem seen yet.
I rack in lot of mileage, and need reliable cars. My biggest mistake so far was buying a bimmer.
My friends, who own a 540 and a new 745i spend more time driving loaner cars than their cars, that's pretty much tell you about Bimmer reliability.
To be honest, even the SL is not as excited to drive as the Bimmer 5 series, however, I believe most people prefer cars that rarely run wrong (or never, like a Lexus), instead of rarely run :-)