Go with the Lexus.
You're looking at 'Most-Reliable brand' VS 'Least-Reliable brand' or pretty close to it. Open your Consumer Reports for proof of this statement.
I only refer to MB's of the last 5 or 7 years (I'm sure somebody had a '92 3-series that went 400,000km etc....) because they are embarrassingly unreliable.
YES! The MB is a luxurious, opulent, impressive piece of work. But you can't admire those traits while it sits at the shop. Maybe, if you're lucky you'd get a little Z3 for a loaner..
Other than mountains of empirical evidence
backing my position, I have anectodal evidence of this: My boss (the only person I know with a MB) had a ML320 he bought used. He liked to say 'It costs about $600 to start it, but then it's a great vehicle!' Because about a half dozen times, he'd park it at night - no problems - start it in the morning, CEL sometimes, once somethingin the engine (can't remember what) broke when he turned the key, other times it ran like crap. After it had cost him several thousand $ in unscheduled repairs over a year, he sold it and bought a used Lexus RX300. He loves it and it runs like a Toyota - predictably.
Oh, this was funny! I dropped him off to pick up his MB after it was repaired. I left and went back to work. He showed up an hour later in a loaner. They had fixed the problem he went in there for, but as he left the parking lot something entirely unrelated went wrong (again, can't remember what.)
Do yourself a favour. Get the Lexus.