My friends stock 5.0 destroyed a dct m3 from 0 till about 130(km/h) where they shut it down.
Its clear that the boss has the power advantage, lets face it for a v8, the m3 has horrible torque. Now for tracking the car, the boss has shown to be faster around the track.
Were all on bmw forums so we can all agree we like luxury and thats where te m3 wins, so OP you need to decide if you want a sports car or a fast luxury car. Now if you dont need the back seats c6 z06 no doubt.
I would personally go with the boss and when you get old and retire than buy yourself a nice luxurious 7 series, for now have fun.