6MT here and as most have mentioned the tracks I've been to are usually 3'rd and 4'th with the occasional 2'nd if I get behind a slower car at a curve.
I've never hit 5'th as the max speed I've hit on the tracks I've been to was 230.
As far as which is quicker. I woul say the DCT there are some places I coul switch down to 2nd but I don't since it would be for such a short period and not worth it but in a DCT it's just a flick of the paddle. I know this because I actually did switch down to 2'nd when I brought my S4 to the track once. DCT is just so easy for the track and even more for inexperienced drivers.
2012 E92-M3 Space Grey on Fox Red, DCT
2011 B8 S4 S-Tronic , Black on magma red