How about the question of carrying 1- double stacked (Glock 19) with no spare magazine, vs. Carrying a single stacked (Kahr K9) with 1 spare magazine? Hmm.
Some number of auto-loader malfunctions I have cleared, over the years, have been best-handled by ripping the mag free. If I am carrying an auto-loading weapon, I am much more comfortable if I am carrying a spare magazine. This is completely independent of how much ammo is “enough.”
I have come to see auto-loader magazines as consumable items. When one is consumed, it is pulled from service, for close examination, then either refurbished with a new spring and follower, or discarded. To help preserve the condition of my carry mags, I do not use them for high-volume training, but even so, I never feel that any magazine is immune to failure.
Having carried a Kahr K9, in the Nineties, and G19 Glocks, more recently, the Kahr is a narrower package to conceal, than a G19, and the blocky rear part of a Glock’s slide is noticeably blockier than the rear of the Kahr slide. This will matter more, to some folks, than to others. I have always wanted to have that spare magazine, regardless.
To be clear, I am not saying whether the Kahr or the G19 is “better.”
My personal Kahr K9 lost favor, when I was no longer able to buy 9BPLE, a then-“LE-only” load, at the local cop shop, and when I started vetting other 9mm +P defensive ammo, my K9 showed itself to be finicky. My wife’s Kahr, OTOH, feeds any and all ammo we have tried in it.