I have owned a E46 BMW and we are currently waiting for the new 435i xDrive M-Sport to arrive.
When we decided to get my wife an all wheel drive vehicle we drove every conceivable option there was. This included the Mercedes C and E class coupes.
This is based on the 4-series comparisons of course.
Our feel of the differences between the BMW and Mercedes come down to design, fit and finish, power and handling.
If you are looking for a "cruiser" with a, in my opinion, dying brand culture, soft suspension, mismatched styling and sub-par power, then chose the Mercedes.
If however, you are under the age of 60 and want to actually enjoy the driving experience...get the BMW.
Hands down the BMW is a drivers car and the Mercedes is a geriatric car headed to the golf course for its morning scotch on the rocks.
But, it's all about personal preference. Buy what you like and remember that a car is worth whatever you want to pay for it. It is, after all, YOUR car.