For me, im just as happy with 9mm or 45, but theres a considerable difference between 380 and 9mm. The 9mm has made great strides in the past handful of years. Winchester Ranger T and HST have brought 9mm to an extremely effective point.... For a handgun.
Research officer involved shootings from Dr Gary Roberts (DocGKR on the forums) and Dr Martin Fackler. 9mm has shown to be almost as effective as 45 with modern ammo. FMJ is a different story.
Now, to get honest, no common handgun caliber is a true man-stopper. Over 80% survive shootings with a handgun, most of which were still able to continue doing what they were doing for a short time after being shot. So, pick the least ineffective handgun ammo and caliber, or to stop a fight, choose a rifle.
Personally, I choose a handgun, then choose the caliber beat for that handgun. Glock, hands down I choose 9mm. 1911, no question for me, it's a 45. M&P, I choose 9mm, but if I needed a 40, it would be an M&P as well.
I carry a handgun for work (as well as a rifle in the car) and am able to choose the handgun and caliber I want to carry. I've studied ballistics exhaustively and have spoken with subject matter experts, well known in the industry that conduct these tests and studies for a living. By all accounts, 9mm is 90% or more capable of what a 45 is with modern ammo. Many departments, including SWAT teams and military special operations are moving to 9mm.
380 is a significant step down from 9mm. It is not close in performance, though I understand why some choose to carry it. I won't, for any reason. But then again, even though I'm not a big guy, I can easily and comfortably conceal a Glock 17 or 1911 in jeans and a t-shirt. Pics are posted in my Flickr account by the same username.
No matter what caliber you choose, your chances of being effective are much lower without proper training in weapons manipulaions, how to employ your chosen weapon, and most important of all, the proper mindset to fight hard, fast and ugly and to win no matter what. Without the proper mindset, NONE of this caliber talk matters.
Get the mind squared away, accept that you may have to take a life, and do so by NOT fighting fair, then choose the handgun that you shoot the best and is most reliable, but that you'll also carry daily. For me, whether a t-shirt and jeans or a suit, I choose a full size handgun loaded with the most effective modern ammo. I understand for some, this may be a 380. That's fine, just train and know you may actually have to use it some day.
The rest is just conversation.
Edit - as those that know me should expect, I'll edit this post with reference material, as usual.
For anyone that hasn't, please read these links.
Dr Gary Roberts on BUGs - http://m4carbine.net/showthread.php?t=19914
Dr Gary Roberts Ammo Recommendations - http://m4carbine.net/showthread.php?t=19887
Best Choices of Self Defense Ammo - http://ammo.ar15.com/project/Self_De..._FAQ/index.htm
I am a 5'4" women and I can easily carry and conceal a glock 17. The biggest thing is practice. I could stop someone with a pitchfork in my hand if I had to. You know what they say.......It's not what you have but how you use it! You have to learn proper placement of that round leaving your weapon. Know where it's going.........each and every time. PRACTICE......PRACTICE........PRACTICE