I normally do not carry a back up gun as my agency does not allow it.
Off duty, I go for maximum concealment, so I use a pocket carry. For nearly a year now, I have been carrying a SIG 365. It has the power of a 9m.m. with a very well thought out design that is probably as small a 9m.m. as I can shoot well.
I tried the micro 9m.m. pistols and found them just too hard to shoot. I could go through 50 rounds and that would be it for me. The SIG 365 is just large enough to be usable, while not especially comfortable. I have switched to HORNADY 100 grain FTX "Lite" jhp ammo as it kicks a little less than the FEDERAL HST 124 grain ammo I prefer in a 9m.m.
For about the 5 years preceding my purchase of the SIG 365, I carried a GLOCK 42. For a pocket carry, it was perfect, except for the .380ACP caliber. It was slim, just large enough for me to shoot it as well as a mid size 9m.m. and had night sights. It was so light, even fully loaded, I would forget it was there and have to check just to be sure.
The SIG 365 is heavier, especially when loaded, so I am always aware it is in my pocket.
In the past, I have also carried a .38 Special, 5 shot S&W model 38 before I switched to a steel framed, DAO model 36. I also on occasion carried a CHARTER ARMS Undercover .38 Special. I was carrying a service issue semi-auto and decided to switch to a semi-auto for off duty carry.
I tried several with varying results.
1. BERETTA Tomcat 3032 in .32ACP. This pistol worked very well for me and my wife after trying it decided she needed it more than I did. I ended up getting a second one.
2. KEL TEC P32 in .32ACP. BROKE THE FIRST TIME I TOOK IT TO THE RANGE. KEL TEC fixed it, but I lost faith in it.
3. NORTH AMERICAN ARMS (NAA) .32ACP Guardian. This is about the smallest .32ACP I have seen and with out a doubt, the most unpleasant to shoot. I limit myself to 25 rounds at a time at the range. The recoil is just miserable. A lot of this has to due with the small size of the gun and the matching small size of the grip. It was much worse than the BERETTA Tomcat or KEL TEC P32. It is even twice as heavy as the KEL TEC.
4. SIG 232 .380ACP, a really great .380 pistol, but simply too bulky for pocket carry. It would print through most of my clothes. On the other hand, it is light (aluminum framed version), has a great trigger for a small gun, very good sightrs and the SIG de-cocker, so no need to worry about a safety. As a belt gun, I would prefer it to just about any 5 shot .38 Special the I have shot.
5. WALTHER PPK in .32ACP. I carried this gun for years. It was small enough that is did not print in a pocket carry, yet shot well. I found it utterly reliable, accurate and just large enough to get a good grip on. Strangely, when I bought a .32ACP WALTHER PP, the slightly large pistol the PPK is derived from, the PPK was easier to shoot. The squared off shape of the PP grip was the culprit and the rounded PPK rear of the grip was much more comfortable.
6. GLOCK 42 which my wife liked so much, it joined the first BERETTA Tomcat as her pistol. I replaced it with the night sight equipped gun I carried before the SIG 365.
7. Finally, the SIG 365.
I used to carry a S&W 681 on duty and it was a big, heavy gun. It was great to shoot at qualifications and would be a first choice for a revolver in gun fight. However, I doubt you will want to carry it all day. I went back to the issued S&W model 13 for off duty carry, because the L-frame was such a drag at the end of the day.
A lighter gun like the 9m.m. with the J-frame as a backup sounds like a better idea.
If my agency would permit a back up gun, I would go with the GLOCK 42. I prefer the SIG 365 as a primary weapon, but I never forget I am carrying it. The GLOCK 42 simply disappears.
Jim