To be honest, I rarely carry a BUG (back up gun). The exception to that rule is when I know I'm going to be in an area with higher risk of a confrontation. When out and about here in town, I simply pack a Glock 26 or 33 on my belt, with an extra mag, and feel safe enough. On the occasions when I do carry a BUG, I tend to go for something small, light, and very concealable. Most often that is either a Smith 637 in .38 Special or a Beretta Tomcat in .32.
Tranter, I see your point about trying to match caliber, but that is not always possible, nor is it always desirable I think. There are always the old problems of concealability and portability to contend with. At what point does one get so bogged down with artillery that he or she can't function normally or even comfortably? Obviously, if any of us knew a confrontation was in the offing, we would probably be packing a short-barreled 12 gauge along with a .45 on the hip! Doing so under normal circumstances tends to cause talk though.