To bring this around to the original question ...
There ain't nothing wrong with a good .40 S&W ... of course a lot is about shot placement, but the
one-shot stop numbers are comparable between the .40 S&W and .45 ACP.
(I've seen other tables that rate .45 a percentage point or two higher, .40 about two percent lower)
But as slang indicated with respect to that cute little derringer ... it's not always about size. A .32 in the hand always beats a .45 in the safe at home.
Frankly, there are times that I just can't carry a full-size weapon, and have to scale down for wardrobe considerations. There was a great article on this in (I think) the December '03 NRA
Women's Outlook regarding how to carry in formal wear.