Are you a troll or is this a real question? For one, the E9X M3 was first available in 6MT and then the DCT came to market. The DCT is great, no doubt, and the 6MT is a proper weighted shift knob away from being a solid shifter.
I'm not going to waste my time getting into the mechanicals of it, but the car was not made for one or the other. I couldn't care less if the DCT is x seconds faster, or gear a bit closer because of the extra top gear. The 6MT suits this car just fine, and I have an absolute blast driving it every day.
There's nothing more to it.