If you're planing on drag racing the car it would make no sense to go with a 6 speed tranny. All the time you waste shifting could add precious 1/10ths to your final time. Remember that every shift you make causes your car to lose momentum, even if for just a fraction of a second. Sometimes that's all that's needed to lose a race. I'm not a big fan of automatics, but one reason they smoke some manual cars is probably for the fact the tranny will usually only shift from 1st to 2nd by the time it crosses the line, whereas with a 6 spd man you might shift right up into fourth.
Just my .02cents