I think this really depends on what you bought your M3 for in combination with your past experience. I have a 6MT. Here's why:
1. My M3 is not my DD.
2. My DD is an automatic
3. I have driven a manual for 30 years, so I am pretty damn good at it. Other than a first gear start from a standstill, DCT gives me no shifting advantage.
4. All dual clutch trannies still all do quirky things. There is no true manual mode in any of them. Total control over the gearbox can still only be had with a true manual.
5. I bought my M3 purely for pleasure. I have no actual need for it whatsoever, so I wanted the car that would simply be the most fun.
6. For seasoned manual drivers, there is just a supreme sense of satisfaction in firing off the perfect shift.
All said, however, if my M3 was my DD, and/or my only car and I did a lot of driving in traffic, I would get the DCT. It has enough pure performance advantages and versatility that nobody can criticize the choice.
Also, my choice of transmission depends a lot on the car. I have been on the M5 boards stating my preference for DCT in that car over the manual. I feel that there is just so much torque being fed to just the rear wheels that DCT really helps you get the power to the ground much better than a traditional clutch and shifter. You can also keep both hands on the steering wheel which I feel becomes more important when you are handling 500 lb-ft of torque vs. 295 lb-ft of torque.
In the end, the choice depends on the car, but most of all, on it's intended use.
Audi S6 * Audi S3 * Porsche Cayman GTS
--Former BMW M3 owner