Ahh .. I remember it well, the ol' "Performance vs Character" sentiment. Whoever said it does come down to personal preference hit it on the head. Yes, the Benz's V8 would out-accelerate the M3, but that's like saying the roadrunner would outrun a cheetah. I mean they're both freaking fast. Both cars performance would be considered "exotic" just 10 years ago. There's no obvious choice here objectively speaking. Buy what you want, plain and simple. To me, there's no such thing as paying too much for something you really (and I mean REALLY) want, but there is a such thing as paying too much for something you don't like.
Let's face it, we would never fully exploit the potential any of these cars have, at least, not in everyday driving. Any who does is irresponsible and would probably wind up in a closet-casket funeral before the first recommended service. What we pay for, essentially, is the enjoyment that we receive to know that we have a car that performs heads and shoulders better than 95% of the cars on the road. Even if we seldom need it to. However, that's no bad thing. It's your money. Who am I (or anyone) to dictate how you should spend it.
As for myself, I won't be able to afford the cars being referred to, but if I could, I'd have to pass on both. There's something called "CTS-V" coming out this fall, with enough horsepower to knock the moon out of orbit. Combine that with the mod-friendly GM V8, and you have a serious weapon on your hand.