Bro, first off go with what YOU WANT man! Sometimes you can get a bit too caught up in what other people say, and go for the majority rules sorta thing...leaving you with the car that you like, but ultimately isn't the one YOU wanted the most.
But if your gonna ask for my opinion
I would hands down say get the M5! Theres a huge difference between the merc and BMW, they're both geared towards totally different styles, driving, and experiences. BMW always has and always will be more of a drivers car, designed more for the experience of driving, touring and tracking and far more performance based overall.
The mercs are more of a flash and drive around town sorta machine, the engineering isn't at all the same and the goals of the manufacturer are totally different...choosing flash, plush leather, etc over performance.
Then again though, I'm a huge car fanatic, I've been racing since I was about 8 and I'm the guy who would choose a lotus elise and specifically have no radio or ac to reduce weight for better lap times then a fully loaded rolls royce...so my opinion might not be the best
P.S. Yes it's a strange combo to be aiming for muscle gain and yet stripping a car to increase performance...lol