The fleet part of the survey (stats from over half a million cars) comes from a trade newspaper about company cars.The number of days their cars are off the road is of course important to companies as are repair costs when out of warranty.This is the most accurate type of survey.
The JD powers private owners satisfaction survey is a bit more subjective, for example people expect more from a prestige brand than a cheaper car.I remember from a few years ago the Merc ML coming one from last out of the 102 models on the UK market.Most other Mercs where mid-pack.On that occasion the BMW 5 series came second and was the only European car in the top 10.
From what I've read I would say Audi and Mercedes are similar in reliability(Audi way ahead on rust proofing though) with BMW some way ahead.
Mercedes do seem to have a rust problem on some 96-2000 cars, a few E class's being quite bad in areas salt is used on the roads in winter.
Check door tops and bottoms behind door seals and around the trunk handle.
Also check the paint quality,sometimes orange peel is quite bad and sometimes the paint is a bit thin around the sills.Although they are well engineered cars you really need to inspect everything and not just assume because it is a Mercedes everything is top quality !

adam