Let me start getting into this by saying not all of us feel the need for a backup handgun. I can live with that, and so can you. Some of us though would be much better off carrying a secondary or backup handgun. For instance a police officer or other LEO, an armed security guard, armed protective personnel (bodyguards), and other people in high risk professions or who live or work in other high risk situations. (Of course by high risk situation I am referring to one that may require you to use a handgun in the first place, I am not talking about something like avalanche danger.) Besides being in a profession or other situation that puts you at high risk, I think a backup gun is not usually necessary; but you may disagree and decide to carry one anyhow. Fine by me.
If you do decide to carry a backup, which should you choose. For instance if you carry a Colt 1911, should you stick with the same brand and carry a Colt Detective Special as backup? Or if you carry a Beretta 92FS, should you carry something smaller as a last ditch backup that is easily concealable like a Beretta Jetfire in .25ACP caliber? Most people to whom I have spoken and who carry backup pieces almost always opt to carry a smaller gun for a backup than is their primary weapon. I think there is some belief that size is of primary concern when deciding which backup to carry. I don't think size in comparison to the primary handgun matters all that much, that is unless for some reason the secondary handgun you are carrying must be extremely concealable.
There are, in fact, a good number of questions you should ask yourself before deciding on which secondary firearm you are going to carry as a backup. Does it need to be, or should it be for any reason, smaller and more concealable than your primary carry firearm? Should it have the same action type as the primary type firearm? Should it be the same caliber as the primary caliber firearm? Should it have the same operating features as the primary carry firearm? Should it have the same or very similar sights as does the primary carry handgun? How many rounds should it be able to hold? What type of speed reloading device does it use (basically magazines or speed loaders)? Should it be able to accept the same type of ammunition reloading device as does the primary handgun?
The answer to all of the above question in the paragraph immediately preceding this one is yes, with the possible exception of the first question. You see the ABSOLUTE best backup handgun you can carry is the same exact type as is your primary handgun. The only limitation on this absolute is if for some reason the secondary handgun must be more easily concealed than the primary handgun, and even then the backup piece is only best if you can answer all of the questions I just asked in the immediately preceding paragraph with a yes about it. More on this a few paragraphs down.
You may be wondering: 'Why is it I should not use a revolver as a backup for my Colt 1911, or why should I use a .25ACP as backup to my .40SW Semi-auto? Or why not use a 2 shot derringer as backup to my .357 revolver. I'll tell you sure you can do that, and you can do it with no problems too, that is if you practice with both a lot. That means practicing with the primary, then going to the backup in practice when the primary jams or runs out of ammo; but it only means that if, and ONLY IF, you are as good under the stress of combat as you are during practice. You see, if you are ever involved in a real life shooting situation, and your primary firearm fails for any reason such as malfunction, or it runs out of ammo, or you drop it, or it takes a shot and is damaged, then you may indeed need to go to your backup handgun in order to survive. If the gun you carry as primary handgun is a Colt 1911 (any variety) and you pull out your backup Colt Detective Special revolver it seems easy enough to continue shooting almost uninterrupted. The thing is if you have to fire those 6 shots in the revolver, and then need to reload, what will you do under the stress of combat. Will you remember that you are not supposed to go to your magazine pouch but rather need to dig out a speed loader, or worse yet dig out loose rounds, or worst of all have not extra ammo for the secondary piece? I can almost guarantee that the pucker factor will by that point have just about overwhelmed your brain by trying to keep your sphincter muscle clamped down on your anus so you don't shit yourself that you will just about be on auto pilot. Whatever you normally do when you run out of ammo in training is probably what you will start to do in this situation. So if you usually go to reload with a magazine, you may very well be grabbing for a magazine to ram into your revolver, muscle memory and training, along with stress, having taken over to some extent. Of course you could get it right, but chances are you will hesitate for just a split second or so, and that is all it might take for you to wind up the loser. So why add the added potential of getting it wrong in the first place, especially if you want to come out the winner?
The chances of you ever getting in a gunfight maybe small, but if you are in one of those higher risk professions or situations I mentioned earlier and need a backup, or if for any reason you have decided to carry a backup, why take any additional risks of having things go wrong? Instead, why not increase the odds in your favor that you will come out the winner in any such situation where you have to depend on your backup handgun! So what does this mean you need to do? You need to assure that either your secondary is the same model as is your primary handgun, or if it is another model than is the primary handgun, the secondary or backup handgun should:
1) Have the same action type as the primary type firearm.
2) Should be the same caliber as the primary handgun.
3) Should have the same ammo capacity as the primary carry handgun.
4) Should use, or be able to use, the same magazines or speed loaders as does the primary handgun.
5) Should have the same operating features as the primary carry firearm.
6) Should have the same or very similar sights as does the primary carry handgun.
After all is said and done, it is not which brand of backup you carry, but rather these mirrored features of the primary and backup that are, in my opinion, what makes it the ABSOLUTE best backup gun to carry. You see, if the primary gun fails you for any reason that forces you to go to the backup gun, the backup will now operate in the same exact manner as does the primary, it will have the same action and therefore shoot the pretty much the same, it will use the same ammo and have the same capacity, therefore you can use the same magazines or speed loaders with it, therefore reloading it in the same exact manner as you would have the primary handgun. The same or very similar sights will have you, in essence, seeing the same sight picture and getting that sight picture just as quickly. If you have to go through some sort of reloading procedure, a malfunction clearing procedure, or making the gun safe, all of these procedures will be the same on both the primary and secondary guns thereby eliminating the increased risk of a screw-up associated with primary and backup guns of different makes and or models with different features. All those things will come into play not only when the SHTF, but also during practice - so by practicing with mirror feature guns you are practicing in an essentially exact manner with each. This can only help assure you get it right under the stress of a a real life situation that requires the use of deadly force.
Now I will admit, the easiest way to do this is to carry two identical pistols or revolvers. For some this may be too cumbersome, and the secondary piece might need be smaller. I will also admit, if you need a second but smaller firearm, there are not many manufacturers who make a larger primary handgun who also make a cloned or very similar backup type gun with the same features as the primary or larger gun, but there are some.
The first such manufacturer which comes to mind is Glock. No I am not a Glock Guru, nor a Glock Nut, nor even a super fan of Glocks. I currently am issued a Glock Model 19 at work, and I have carried a Glock Model 26 as backup for it. I also carried a Smith & Wesson Model 60 as backup for the Glock 19, but using my own logic I decided the Glock 26 would be much better of a choice, and I changed as soon as one became available. When I started doing admin work some years back I gave up the model 26 so that another agent in a higher risk group could get one. Sadly I am again in a higher risk group, but no Glock 26s are available. All of the mirror operating features I described work between the Glock 19 and Glock 26. They are both semi-auto pistols, they have pretty much the same sights (or both can have the same sights), the operate exactly the same, both are 9mm Luger caliber, both have the same ammunition capacity, both can use the same exact magazines... 'Oh wait a doggone minute...' you say, '...the Glock 19 holds 16 rounds and the Glock 26 only 10'. Nope you are out and out incorrect by a technicality. While the model 26 can use the 10 round mags, and the 19 cannot use those smaller mags; the truth is that both of those pistols can use the higher capacity 15 round magazines that come with the Glock 19. Therefore both can have the same capacity as to number of rounds. So you only need carry spare 15 round magazines, in fact if you carry both of these pistols at the same time I strongly state that you should not carry any 10 round magazines with the possible of exception of the magazine actually seated in the Model 26. All backup mags should be the 15 round capacity mags.
Other manufacturers have similar pistols. Beretta has the 92FS, and has or at least had, a smaller 92 compact model. I believe Sig has some similar pistols in different models, and some in the same model but just of different size such as the Sig P220 and the P220 Compact. Smith and Wesson has some models of revolvers that will use interchangeable speed loaders. Colt probably made revolvers likewise.
So what is boils down to is, in my opinion, the ABSOLUTE BEST BACKUP HANDGUN is one that has functions/features that mirror the primary carry handgun if only because of the lifesaver that this can be in a firefight.
All the best,