Caliber is determined by the gun I choose.
9x19, could be in the P226, CZ 75, 92FS, 915 or DP 51 (the Daewoo is the smallest of that bunch and a fine shooter, but I've open-carried the others after bad storms when the power is out).
9x18 Mak, I have a CZ 82 that's a nice little package, and a good shooter. Often with me in my car. And a P64 that's small enough to stick in my pocket.
45 acp, when the urge for a 1911 grabs me.
32 acp, I have that Beretta 81 with 12 in the mag, it's a fine shooter. Also have a little FEG PA in that caliber, my version of the Walther PP if I want to be all smooth and Bondlike with a shoulder holster.