I'd say how about a S&W MP40 with 15 rounds, then you know your not to run out even if you miss a few.
The MP looks like a fine gun, but its just too damn big.
as the other poster said just buy the extended mag for the .40 if you are worried.
The 9mm comes with a +1 round mag too, to the question remains vaild; now its just 6 vs 7 is all. Im not worried by the way, just interested in the opinions of others.