That's a very difficult question to answer. I think it depends upon what your area of interest is. But since this is the CCW forum, I'm guessing you mean setting up a collection along those lines.
And in that case......
I see it broken down into three areas- Primary Carry Gun; Backup/Always Gun; and Alternate Primary Carry Gun.
1-Primary carry gun:
Whatever that may be for you. I carry a full-size handgun year-round, but others go for a mid-size "compact" or "pocket" gun. I'll go with what I use here.
I usually carry a 5" 1911. Therefore, I would always want to have at least two of those, set-up at least fairly close to the same. If one goes down for some reason, I don't want to be waiting on parts or a gunsmith.
For the past couple of years, my primary has been a Kimber TLE stainless, with a lightly customized Colt 1991 "standing by". But I have no shortage of 5" 1911s.
2-Backup/second/always gun:
I always have a smaller gun, usually in a pocket holster. I have the primary gun 99% of the time I'm awake, but if not, I still have the #2 gun.
Besides the reasons usually given by people for having a "backup", it's also handy in case I have to loan a gun to my wife.
I used to always carry a S&W J-frame here, but switched to a Kahr PM9 Covert about 18 months ago.
3- Alternate Primary Gun:
Sometimes, due to what I have to wear, I don't feel like I can conceal a 5" 1911 that well. I'm probably OK sometimes, but don't want to take chances. I don't really want to just go with the backup gun only. In those cases, I go with a slightly smaller full-size or compact gun. Although i usually carry a 5" 1911, I don't to a compact 1911 here because I've had less-than-ideal luck with them in the functioning area. I prefer to just go to a different design.
Here, I use a Browning HiPower or HK P7M8.
The HiPowers aren't that much smaller than a 1911, but sometimes a little is all it takes. The HK P7 hides under almost anything.
There is a sort-of system going on here.
The "Primary" and "Alternate Primary" guns all work in similar ways. I'm not switching from single action, to double action, to safe action, etc everytime I have to change guns. Going from the 1911 to the HiPower is a pretty easy switch due to similar trigger pulls, control locations, and so on. The sights on mine are basically the same.
The HK P7 is not as close, but the trigger is very similar, and is not a bad transition at all.
The Kahr backup is about as different as I'd like to go here. To me, the grip is Hi-Power-like, so is familiar. The trigger, while a long, DA-ish type, is easier for me adapt to after SA autos than a Glock auto or S&W revolver.
If I were still carrying a DA auto as a primary, like my Sig P220, I'd probably go with something like-
The P220 as the Primary, a P225 or 228 as Alternate Primary, and the Kahr for Backup.
Wow, that was a long-winded answer, and maybe not what you had in mind anyway, but there it is.
