Rastoff
US Veteran
With proper training this becomes moot. Your follow up shot can be just as fast with a .40S&W as it can with a 9mm....additional fractions of a second needed for accurate .40 follow up shot...
I would be willing to take anyone here to test this. We could start with the 9mm as the bench mark by using a timer. Then do a little training. Then shoot with the .40S&W. Within a day I'm sure I could get most people to shooting two controlled shots, on target, within a hand span of each other, with the .40S&W, as fast as the 9mm +/- .1 seconds (that's as much resolution as my timer has). In fact, if we start with the 9mm as a bench mark before there is any training, I'll bet you come out faster with the .40S&W.
The point here is, pick the caliber you're most confident with. Speed will come with training and practice. In my opinion, whether or not the .40 has more power is irrelevant. You must get good hits first.