Here is an excerpt of an article posted here at Concealed Carry Forum about ammunition choices for handguns.
This has to be the most heavily debated topic of any given gun related forum. Starting as "9mm vs. .45" during WW2, with many new caliber choices the caliber wars continue to rage. Some believe in big bullets...some believe in lighter, faster bullets with higher simple energy numbers...some believe in shot placement....all are equally mistaken. The potential effectiveness of any bullet in a defensive shooting is always an issue of probabilities that factor in caliber, weight, velocity, shot placement, (resulting in penetration and expansion), and even the physical condition (drugs, rage, size, etc.) of the threat you are attempting to neutralize. You will hear of threats dropping immediately from a .25 ACP and you will hear of threats that continued to fight despite a solid .44 Magnum hit but these are rare exceptions and should never be considered representative in any way of overall stopping power potential. They are nothing beyond extremely rare incidents that have minimal impact on the overall averages used to calculate overall stopping power potential.
If you remove the human element (your ability to shoot, for example) and limit your comparison to bullet performance its hard to argue with "bigger is better". Despite lower simple energy numbers, bigger and particularly heavier bullets carry more stopping power potential and this is clearly substantiated in history.
Just as it had in the previous century, the .45 Long Colt proved itself by dropping the Moros Tribesmen most often with a single shot (yes, a single torso hit from the .45 Long Colt proved superior to 5 and 6 shots from the .38 Long Colt!). This is documented US history and there is no disputing it. The .45 Long Colt played a crucial role in winning the war, and this directly lead to the development of the .45 Automatic Colt Pistol (.45 ACP) which served our troops equally well through two world and many smaller wars.
In and of itself with all else being equal (shot placement, etc.) hitting a threat with a 230 grain .45 caliber slug is not the same thing as hitting that same threat in the same place with a 115 grain .355" slug despite its considerably higher velocity. With typical handgun velocities at typical defensive handgun ranges bigger simply is better....but how much better is the question, and at what cost? 20 years ago there was significant difference in ballistic performance of the 9mm compared to the .45, but modern ballistic science has brought the 9mm closer to the .45 than ever before. While I still believe bigger is better (you don't shoot elephants with small and fast bullets, you shoot elephants with big, heavy bullets), it's hard to argue with a 147 grain 9mm bullet that penetrates 14" and has been shown to expand to .72" (147 grain federal HST).
Although true and accurate, does the example of the Moros Tribesmen mean that everyone should carry the .45? Absolutely not. Although the .45 would serve anyone very well just as it has throughout history, this does not make it the superior choice for everyone. .45 caliber weapons are either relatively large or they have very limited capacity and this limiting factor rules out the .45 for many. To confuse the issue even more, although I firmly believe that all things equal bigger is better, all things are rarely equal when you factor in the human element. Although the 180 grain .40 offers slightly more stopping power potential than a 124 grain 9mm that does not necessarily make it a better choice for carry due to many reasons. If you find the .40 recoil uncomfortable you probably wont be able to hit with it with as much confidence as you would with the 9mm. Capacity is another issue, especially when you factor in cover fire. Is it better to have 8 rounds of .45 in your weapon or 16 rounds of 9mm? The answer to this question will be as unique as the individual in question. The following is a comparison of caliber with my assessment of each round:
.22 LR
Good:
Weapons are small.
Ammunition is cheap and plentiful.
Adequate as a backup weapon.
Bad:
Not adequate as a primary weapon.
Notes: - If you carry this caliber for a backup weapon, carry CCI Mini-Mags.
.25 ACP
Good:
Weapons are small.
Better than nothing.
Bad:
Not adequate as a primary weapon.
Not adequate as a backup weapon.
Ammunition is extremely expensive.
Notes: - Please don't carry this caliber. I have literally witnessed the .25 ACP knock over Coke bottles at 50' instead of breaking them.
.32 ACP
Good:
Weapons are small.
Better than nothing.
Adequate as a backup weapon with FMJ ammunition.
Bad:
Despite its relative popularity, the .32 ACP is not adequate as a primary weapon.
Ammunition is relatively expensive and choices can be limited.
.380 ACP
Good:
Weapons are small.
Adequate as backup weapon with FMJ ammunition.
Bad:
Despite its popularity the .380 ACP is not adquate as primary carry weapon with any ammunition.
Ammunition is relatively expensive.
.38 Special
Good:
Adequate as a primary carry weapon in 158 grain +P loads.
Adequate as a backup weapon at standard pressures.
Compact weapons.
Economical to shoot.
Very popular caliber with many great choices in weapons and ammunition.
Bad:
Limited capacity (revolver).
Marginal when loaded at standard pressures.
9mm Luger
Good:
Excellent as a primary weapon in 147 grain loads or 125 grain +P/+P+ loads.
Ammunition is cheap and plentiful!
Most popular handgun cartridge in the world offering a nearly unlimited selection of weapons.
High capacity even in compact designs.
Bad:
Marginal performance from light loads at standard pressure.
Notes: - The 9mm Luger has been around a little longer than the .45 ACP and has attained unmatched global acceptance. Most every modernized nation in the world today issues combat side arms chambered for the 9mm Luger. Because the 9mm is considerably smaller and lighter than the .45 the weapons chambered for it are able to carry more rounds, as are the soldiers who carry them. 147 grain loads make the 9mm a truly viable defensive caliber and this is the ammunition I recommend if your weapon will cycle it reliably. If choosing 115/124/125/127 grain loads, carry only +P.
357 Sig
Good:
Excellent as a primary weapon.
This round just sizzles - 125 grains @ 1450 fps!
Penetrates hard objects like no other handgun round.
Bad:
Ammunition cost literally twice as much as 9mm.
Ammunition selection is limited in most areas.
Generous recoil (relative).
Notes: - Were it not for cost, the 357 Sig would be the best recommendation I could make for carry. The US Secret Service chose this round to protect and defend the President of the United States.
.357 Magnum
Good:
Excellent as a primarly weapon.
Most versatile handgun round with huge ammunition selection.
Can practice with cheaper .38 Special ammunition.
Bad:
Limited capacity (revolver).
Generous recoil (relative).
.40 S&W
Good:
Excellent as a primary weapon.
Economical to shoot.
Large ammunition selection.
Bad:
Snappy recoil (relative).
Notes: - Offering the perfect compromise (without compromising performance) between the 9mm and the .45 ACP, it's quite hard to find fault with the .40 S&W. The .40 was born indirectly due to LE rejection of the 9mm after the famous FBI shoot-out in Miami in 1986. Offering more stopping power potential than the 9mm and more capacity than the .45, the .40 fills the gap between the two perfectly. You really don't give up much stopping power potential compared to the .45, and you aren't losing much capacity compared to the 9mm. The .40 is the ideal carry caliber.
.45 ACP
Good:
Ballistically superior as a primary weapon.
Big, heavy bullets.
Effective even in FMJ.
+P not required for superior performance.
Abundant ammunition selection.
Bad:
Weapons are either large or have limited capacity.
There are other calibers worth mentioning because they are viable defensive rounds, yet I don't typically recommend them for various reasons.
Super .38: While ballistically superior to 9mm, the choices in the Super .38 platform (both in weapons and ammunition) are few and far between. Dealers who do stock ammunition offer a very limited selection and what they do have is extremely expensive. The Super .38 is a handloader's caliber.
10mm: Ballistically the 10mm has stopping power potential matching (and in some loadings exceeding) the .45 ACP. Though the 10mm is thought by many to be the ultimate caliber, instead of expanding this platform the 10mm has been downloaded to the point that most 10mm loads available are only marginally better than .40 S&W while costing twice as much. There are a couple decent 10mm loads out there but they most always require placing an order and waiting. Like the Super .38, the 10mm is a handloader's caliber.
.41 Magnum: The .41 Magnum is a beautiful round with beautiful ballistics that never found the glory it deserves. Although it would serve anyone very well, industry support for this round is dying.
.44 Magnum: "The most powerful handgun in the world" generates some very impressive ballistics but I don't feel weapons chambered for .44 Magnum are (typically) practical for carry.
There is an almost infinite number of calibers not mentioned, but these are the only ones I would even consider for carry.