I've owned a Mercedes and I've owned 2.5 BMW's (two proper ones and a Mini with a BMW engine in it). All I can say is that the Mercedes wins hands down. There is something about the power delivery that makes it a feel a lot more special. The BMW's are workhorses in comparison but workhorses with poor quality hearts.
Saying that I've not owned an AMG or proper M BMW. The 330ci with the M54 engine is is a close as I got.
Car of the year / decade etc awards are given by journalists with no long term experience of running the vehicles concerned, so tend to focus on the superficial. Of the two models, I prefer the Mercedes, but not enough to buy one, then nor pay the risible prices asked for what are now mediocre performers. Cars developed to perform on the track, make lousy day to day transport, and high revving four cylinder engines suit motorcycles more than cars. Today I would steer well clear of both marques, having poor personal, family and friend experience.