I already own a .45 ACP; while it's great for home defense I can't imagine lugging it around on my belt all day. So I've been trying to decide for myself whether to purchase a .380 or 9mm caliber handgun for carrying purposes.

I found this terrific article by Dick Metcalf, who's the Technical Editor at Shooting Times:

When it comes to personal-defense carry, most people buy little guns. The single largest category of handguns bought in the US during the last 10 years has been small, short-barrel, pocket-size defense models-autoloaders and revolvers alike. Overall, compact concealment-size handguns account for more than 70 percent of all current civilian handgun sales, and autoloaders account for approximately 75 percent of that number (according to the most current BATF statistics). In terms of caliber selection (not counting the sub-effective .22 and .25 chamberings), the two most popular choices within this dominant portion of the overall handgun pie are the .380 ACP and the 9mm.

My research has helped me settle on a 9mm.

[9mm Versus .380 ACP For Self-Defense]