It is hard to argue against quantity.
My preference for the .45 is because I shoot 1911s better, one handed, than I shoot other autos. (Stances are great at the range and in competition, but Murphy's Law suggests I may not have both hands available for the gun, in a fight.). I usually carry 2 spare mags with a 1911, giving a total capacity of 25, but also importantly providing mechanical redundancy and some counterweight to the pistol.
The CZ75 family come in second, one-handed for me, so my primary 9mm is a P01. With one spare mag to add capacity, redundancy, and counterweight, that's 29 rounds.
My really small 9s offer no capacity advantage over a 1911; the MK9 actually holds fewer. Then again, they typically serve me as BUGs on my weak side.