Howdy,
I'll look at this from a different point of view:
Why, as a civilian with a CCW permit do you feel the need to carry a backup gun when there's a 99.99% chance that you can CCW for 50 years and never need your handgun.
I probably CCW for a different reason than most people on this forum.
The reason I CCW is because I can. The USC and the State of Arkansss says that I can carry a handgun on or about my person so, being a hardcore gun nut, I do.
I'm not worried about being robbed, carjacked, mugged, etc. so I carry because I want to and because I can.
Pretty simple.
Back to the OP question: I have a Kel-Tek PF-9 9mm that I've fired over 1,500rds through it without a hiccup and if I wanted a small BUG it would be my 1st pick because it is very small, thin, lightweight and even though I'm not a 9mm fan it has more stopping power than a 32, 380 or stubnose .38.
HTH
Paul