BMWs Mercedes' are totally different cars, and i say that not meaning to slag either of them off...
firstly, i believe BMWs are more sport oriented, and Mercedes' more luxury oriented - most BMWs come in manual guise, mercedes dont - i think that says a lot in itself.
BMWs pride themselves on hi-tech, lightweight engines - straight sixes to deliver smothness, power, and a lovely note. Mercedes are happy with low-tech (they still use a single cam in some of their engines) crude and harsh engines, and supercharge the crap out of anything to gain power - not the most elegant engineering.
Mercedes (or AMG rather) seem to have the belief that bigger is better - shoehorn a massive engine into anything to make it go faster - ie c55? wtf? a 5+ litre engine in a small sedan... that is freakin stupid. slk 55? ditto. sure they go fast... but is it really all about that?
Don't get me wrong, Mercedes have lovely interiors - nice leather/wood... beautifully put together and finished... and i'm sure there as safe as fort knox.
but I think i've made my point clear that bmws and mercedes are different enough car brands to appeal to different people. neither being better than the other - their primary focus, i think, is totally different.
calling a bmw a 'poor man's merc' is ultimately, an ill-informed statement with no substance behind it.