Say a person is going to own just one pistol? Some would say a full sized gun is the way to go but I don't like this option. The reason I do not like it is those guns are generally not very concealable. Sure they are great to shoot and as house guns but the strength of the pistol is as a gun you could carry. In general I believe if you are going to have just one hand gun it should be concealable yet also something you can fight with. Compact handguns like the Glock 19 are usually considered the way to go. I wonder if this really makes sense.
A sub compact handgun will carry better which means you are more likely to carry it. Sure many folks carry compact or even full sized pistols but most of those are pretty dedicated folks. Most folks serious enough to be packing a full sized pistol regularly (not once in a blue moon) tend to have deeper collections anyway so that point is moot. For a relatively new or fair weather CCW type a subcompact pistol is going to get carried a lot more.
In a situation where the subcompact handgun falls short odds are I will be carrying a rifle anyway. If my modern semi automatic mag fed rifle falls short or empty I would transition to the pistola. While not optimal a little mini Glock or the M&P equivalent is still pretty darn handy. The negligible difference between say a G26/27 or a 19/23 is probably not going to be a difference maker and even so the advantages for concealed carry probably out weight that.
Sure a pair of handguns is better. One for CCW and one full sized SHTF/ house gun. A baby glock and a full sized one that takes the same mags is a good option. Also a couple guys I know have little .380's and full sized .45's. Caliber and logistical complications aside I think that is a nice set up.