In my opinion there is no question, go for the manual transmission. I *ONLY* got STEPTRONIC for the Mrs. In 2009 when I got my 135, BMW didn't have the DCT option for a 135 yet, so if you wanted paddles you needed to go STEPTRONIC. Since I needed auto for the wife, I was definitely springing for the paddle option. In DE events, STEPTRONIC is insanely easy to drive, which does allow you to focus on other aspects of your driving (line, braking, etc.), but I'll be happy when my dedicated Porsche track car with a proper manual transmission is done to flog that out on the track.
IMO, BMW may be the "Ultimate Driving Machine", but with all the electronics they keep adding to their cars they are effectively taking the "driver" out of the car.
Again, this is just my opinion. I am not looking to start a "religious war" here. Its my thread so my opinion. If your opinion differs, great, more power to you.