If you think caliber is the question or problem, you simply don't understand.
38,357mag,9mm,357SIG,40S&W,10mm,45acp. And few other more esoteric caliber (38super, 9x23 45gap etc....) have history of being RELATIVELY effective. And in every case a poor substitute for a rifle round or if close range, a shotgun with at least #1 buckshot as a minimum.
Make sure your weapon fits you. You should not have to adapt to fit the gun. Most guns either fit or don't. today, gun makers are now making weapons that can fit several different size hands. It is one of the reasons the 1911 is and has been so revered. It can be made or adapted to fit almost any hand size. For over 100 years Saint Browning was ahead of the curve there too.
Choose good bullets in what ever caliber was the caliber your platform/gun of choice was designed for.
Being reliable is the number one through three reasons to choose any weapon.
Quality training, and a lot of it. Quality practice, Maintenance, cleaning, and lubrication as needed.
The fellow who I am referring to below, is the leading researcher in "stopping" power in small arms in the United States. The FBI use him quite often.
http://m4carbine.net/showthread.php?t=19887
If you take a handgun to a gun fight, Darwin has a place for you. You get caught in a fight with a handgun, you go to a gunfight with a shoulder arm, unless you have a strong need to reinforce Darwins theory's.
Go figure.
Fred