i almost always carry a CZ75 of some sort, usually a P-01, and a full-size back-up mag. a P-01 is roughly the size and weight of a glock 19 and holds 14 in the mag. it's comfortable/light enough to go everywhere with me and disappears on my hip even with just shorts and a t-shirt.
if i have to dress up and tuck a shirt in, i carry a bulgarian makarov on my ankle. 9x18 isn't the best defensive caliber, but it beats a blank . . . .
as for caliber choice, any caliber that will push a HP in and out of an attacker is sufficient (12"+ ballistic gel). 9x18mak will not, so i carry FMJ in that pistol. i really need to get a 9mm pocket gun . . . .
frye