My opinion and impression ...
A Beemer 5 series is for businessmen who are at the climb of an averagely successful business. A more "in" and daring man. Have you seen the BMW X6... no, not X5 or X3. X6 is beautiful... (It looks better in real-life than in the website. Sporty and cool.)... Also M6 convertible... the Z4... etc Check it out.
http://www.bmw.com/com/en/newvehicles/x6/x6/2007/visualizer.html
My impression of driver of a MERC S Class...(MERCs are out and boring for me except for the convertibles which I love)
MERC S280 is for old boring business men who rather play on the safe side or for rich grandfathers, that also, driven by personal drivers. If you are thinking of a second hand of an old model of a MERC and you are a businessman and if this is your first car, then it'll be quite a turn off. And this may reflect on your business. Have you seen the MERC SL... check it out. I love it... got some space behind instead of just the 2-seater SLK.
http://www.myride.com/review/2008_mercedes-benz_sl-review-3675-page1.html
My advise to you... if you are not a grandpa yet... get the BMW.