This is a good topic and a real can of worms. I want to open it a little further. I am speaking here only of what I have found to be true, if you do not agree with me more power to you, you are protecting your own life and should do that however you feel is best. With that said I found myself in an alarming trend. At first I thought a 9mm glock was a good carry gun (and it is), but then I figured why not get one of those nifty SW airweights and carry that? So I did. My logic was that I probably wouldn't need the gun but it was nice to have something on me anyway just in case. I had forgotten that if I ever did need a gun I would need one that could deal a decisive blow and not just one that could make a loud noise. I took this another mental step and actually thought of getting one of those neat 22 walther TPH's and carrying that. Had sights all picked out and everything... Then I had an interesting conversation. I am active duty navy, this took place about 5 years ago when I was in the Defense Language Institute in Monterey. We have a lot of SEALS roll through there and I was talking about guns with one of them. I asked him what kind of guns he had? He told me he just had one at home, a glock 21. I asked him why just one and he said that if he only had one then he could only practice with one and he'd be real good with it. I asked him why .45? (This is his actual answer, I apologize for the abruptness of it

"Because I shot some guys with 9mm and they all kept coming, I shot a guy with a 45 and it dropped him like a sack of shit."
So that decided caliber for me. I now think that all guns are fine and interesting tools, but the only ones I want for defense are .357, 10mm, 45 and like powered rounds. I like the 45 but that is very subjective. I also feel that the ability to place hits is more important than the caliber. For that matter I feel that the ability to sense that something is wrong ahead of time is better than all the above put together. I am still working to develop those skills.
Now, finally, to answer your question: I have nothing against back up guns. If you carry one then more power to you. My thoughts are that if it isn't a large caliber pistol that I would be better off with an ASP baton or a large flashlight. Here is my logic on this: Criminals are not brave and they are not looking for a fight, not a fair one anyway. If they come at me it is pretty good bet they are in a group. Cowards feel safer in a group and criminals are cowards. If I have a .22 or even a .38 I can do some damage, but I doubt the ability of these guns to take away a criminals ability to hurt me or my family. If I ever have to shoot someone I figure that a .45 with 9 rounds is good for 3 guys maximum. I worry about being able to reload fast enough with a 1911. With a 5 shot j-frame I figure I can deal with one or maybe two bad guys. The people I see that look like they would maim me for 10 dollars are usually traveling in groups of at least 3, usually 4 or more. I would never shoot someone to kill them out of spite. What I mean by that is that my goal is to remove the criminals ability to harm me. It does not help me if he dies 2 days later, or an hour later or even 3 minutes later. My goal is to remove him from the fight so he can not hurt me or my family any more RIGHT NOW. So, for me the answer is that I feel I can do that better with a large flashlight or mace or an ASP than I can with a .22 or .25 or 32 or .380.
Do I want to get shot with one of those guns? No. But I do think I could still cut someone up pretty badly after they shot me with it. I might die anyway, but what do I have to lose by continuing the attack at that point. I am angry and still very mobile and able to inflict damage. So for me, and I am only speaking for me, I prefer a second large bore handgun or a mechanical means of defense. For me it is not a knife because I do not know how to keep someone from taking it away from me and hurting me with it. I carry a folding knife but that would be way down low on my list of options. If I am attacking with a knife it means I am running away and someone tackles me and wants to still fight and I have no other means of resistance.
For me the best back up gun would probably be the glock 36. It is slim and has no manual safety on it. I can work it simply with either hand.
But, I am a civilian. I avoid danger when I can. If I was a police man I would carry two or even 3 guns on me if I could arrange it. Anyway, I would rather use up my 1911 and transfer to a collapseable baton than switch to a mousegun.
Okay, that was long. Sorry. I am now putting on my asbestos long underwear... Okay, gentleman - have at me!
Good shooting. Jake