If I knew that I was going somewhere where I would possibly be engaged in a deadly confrontation with firearms, then I just simply wouldn't go there or if forced to I would then take a shotgun or an AR-15. Nonetheless, I trust my training and current handgun rotation which consists of an H&K USP Compact .40 S&W (12 round capacity), a Walther P99 9mm (15 round capacity) and a S&W 442 .38+P (5 round capacity) to handle any situation. If I were to go to a "more dangerous" area than normal, then I would take a backup gun and plenty of ammo (2 mags per gun). I'd probably go with my H&K USP Compact .40 S&W as main carry and my Walther P99 9mm for backup.
Nonetheless, I think that shot placement is more important than caliber. Don't forget the massacre that psycho Seung-Hui Cho delivered at Virginia Tech in 2007 with a Walther P22 in .22LR and a Glock 19 9mm. He killed 32 persons and injured many others with a very questionable caliber (.22LR) and the other is what some forum members consider the minimum (9mm). I don't support the massacre or agree with the perpetrators actions, I am just using it for reference so don't take it the wrong way. I despise the attacks at Virginia Tech!