This quote from the article sums it up for me.

"I’d say that even though the Mercedes has more power, the M5 is a more thrilling ride. It really does feel very special when you’re in Exit mode, being suicidal. But I hear horror stories about the BMW’s electrics, and to live with every day the more refined, more grown-up Mercedes — I can’t believe I’m saying this — is the better bet."

In fact, this statement can be applied to almost all the AMG and M cars. For everday driving, the AMG cars are easier to live with, while the M cars rule on the track.