Depends on what I'm wearing.
My two primary CCWs are .45 and 9mm.
If I'm carrying a backup its typically 9mm, .38 (yes snubby), or .22.r (so small my zippo in my pocket hides it.)
As a non-LEO the odds are higher I won't be running out of ammunition and the bad guy gets the first move so a backup odds is more of if your primary is taken. As far as a hide gun goes, this is both good and bad.
Where an LEO the badguy will almost for sure check his ankle, the odds are lower they won't on a non-LEO. An ankle holster doesn't too well when wearing short pants. That's where a nice pocket gun comes in handy.
Of my small guns, I favor the G26 and .38 snubby.
I'd have more but I have pretty restricting criteria for small guns which is a hammer block/drop safety system but still one trigger pull use. This limits my selections.
Tj
edit: BTW, Some of you guys have some classic combinations there.