If you want a true comparison you must repair MB problems with MB fixes not BMW fixes. Yes, on a BMW one must take the head off and might as well do a whole valve job to solve valve guide oil seal problems (a quart of oil every 500mi). But this is just a waste of time on a MB whose seals are almost as easy to replace as spark plugs.
My experience is so vastly different than yours. I have worked on both car lines for almost 30 years. my experience is also weighted with personal considerations. My experience is both as a technicain and as a driver. I have owned probably 50 MBs, ten as personal drivers. I have owned 2 BMWs (so our experience as drivers is reversed). The first was a red 2002 that I bought for my second daughter's 16th birthday (it was an automatic and as most BMWs of the day had no performance that I noticed).
I currently drive a E39 540i BMW and there is no better driving car produced by MB. Power is comparable maybe even surpased by the various V8 "E" chassis, but the handling isn't close.
I have owned the car for 2.5 years and I can say I have never owned a vehicle that made me pay so much for the pleasures it gives. Radiator, water pump, electric thermostat (probably took out the radiator - the water pump was later). The car has 54k on it now and these failings are premature but could be considered aberations except for all the cheap POS problems that are occuring. Six months ago the little flap that covers the rear window child lock out as part of the main window switch, fell off. Its part of a $250 switch package that has to be reprogrammed when replaced. The reason it failed was its design had it work by flexing a hinge point made ofthis plastic. The previous owner must have sat there pushing the button up and down as I would never even use it (I don't haul people in the back seat - we take the wifes MB). Next after detailing I notice the right side colder temp button on the $500 A/C assembly (programmable of course) has disappeared. I probably have never touched it.
Early in my ownership I had to machine plastic headlight adjusters when the originals (only available as part of $800 assemblies) turned to powder and my headlights bobbed. As a direct result of my BMW ownership experience I have initiated programs in our business to do more BMW repairs. I mean that from both aspects; both as a wonderfull driving machine and as one needing countless costly repairs.
Bosch Master, ASE Master, L1
33 years MB technician