*Long sigh*
These caliber discussions will never end. OP, the best advice I can give is to carry the handgun that you're most comfortable and PROFICIENT with, regardless of caliber.
Now, with that said... you should aspire to attain those two things I mentioned with the biggest caliber you can. If it's 9x19, 10mm or .44mag, whatever, just get those two things nailed down.
As Chen was so kind to mention, basically, handgun rounds SUCK. Unfortunately, it's what most of us have to work with when going about our daily lives in public. There isn't sufficient energy available to be transferred to our target to count on any temporary wound cavity reliably disabling our foe (outside say, .44mag+ options). The permanent wound channel is our only savoir to rely on. Sometimes people get lucky, sometimes they don't... maybe your attacker will flee at the first sign of a confrontation or maybe the first round being fired will do it. Then again, maybe not.
Most people who get shot with handguns live to tell about it, thanks to seeking/receiving medical attention soon. That means, most of them were not permanently disabled or killed in the gunfight. Although, perhaps them being temporarily disabled was enough to allow the other guy to flee and live. That could be enough to save your life during a confrontation.
To finish, Chen nailed it when he said that more wounds in more places is better than fewer in fewer places. Unless you score a hit in a critical place like the CNS or a critical bone structure (think: shattering a pelvis), your attacker is going to be conscience and able to fight. Unconsciousness due to blood loss takes valuable time, sometime dozens of minutes, depending. The more leaks you can create (hopefully in the 'better' places), the more likely it is that your attacker will cease their attack sooner.
Train, train, and train some more. Train in as stressful an environment as possible, from every conceivable scenario. Pay attention to your surroundings, be vigilant and willing to do harm when it must be done. When it comes to that, be relentless in your resolve to win the fight.
