6MT here. Just depends on the experience that you are looking for. I know that DCT would be easier (and safer due to the possibility of money-shifting), and probably worth at least a second around most tracks that I go to, but I love blasting off those cleanly executed heel-toes before careening around corners.
It's fun, I love it, and I don't get around tracks all that much slower than most.
However, for people who aren't used to heel-toe, rev-match, double-clutch, etc., it is a whole lot of work to do when you're already faced with a lot of things to do. I was sitting in my friend's car, and it was almost dangerous in the ride along that I got: since he's incapable of all of the above, he'd just drop a gear, let off the clutch, get the synchros to do the work, and then get around the corner. Problem is, he'd sometimes do this too late going into a corner, and it'd really upset the balance of the car when cornering, since the car jerks. Thank God for WRX all-wheel tech.
Anyways, depending on the track, I typically use 3-5. There's one corner at Buttonwillow which is a bit tricky for the M3: I'm still not sure if I want to use 2nd gear there or not: dropping into 2nd I run out of RPM's too fast, but trying to push out in 3rd you run into the M3's lack of torque issue at around 3k RPM.