We've all heard the sayings: "modern 9mm is as good as 45", "45 is a proven caliber, it won us two world wars!", "I'd take 15 rounds of 9mm over 7 rounds of 45 any day", or my personal favorite: "9mm kills the body, but 45 kills the soul". While this debate (and the assorted phrases) will certainly carry on, I think it is worth having the conversation at least one more time.
So, what is the difference between 9mm and .45 ACP?
9mm Luger (also called 9x19, or 9mm Parabellum) was invented by an Austrian firearm maker (Georg Luger) in 1901. 9mm has been common for semi-automatic handguns for decades and is the standard handgun cartridge of NATO countries. This makes it very common, fairly easy to find, and relatively affordable. Having a relatively narrow diameter, one of the greatest benefits of 9mm is ammo capacity.
45 ACP (Automatic Colt Pistol) on the other hand was invented by John Moses Browning (yes that Browning) in 1904. It uses a wider and heavier projectile than the 9mm, and was adopted as the chambering for Colt's 1911 pistol. Being a slower round, 45 lends itself particularly well to being suppressed (since most loads are naturally subsonic) and it's comparatively low chamber pressure are believed to extend weapon service life.
While that statistical differences between the two calibers are obvious, the question that is often asked is "which should I carry?". My answer to that is simply: it depends. Let me now make a case for both:
9mm is the most widely used handgun caliber in current service on the planet. It can be found in military armories, police barracks and in just about any serious gun owners gun safe. That makes it extremely easy to find, and more importantly, relatively cheap to buy. What's more, guns chambered in 9mm often can hold substantially more rounds than those in 45, which means if your life is on the line, you have more opportunities to stop the threat. Also, the relatively light kick of 9mm makes it somewhat easier to shoot, which can be big factor for any shooter, but is especially important to the new shooter. 9mm firearms are also traditionally smaller, which for concealed carry is a concern for everyone.
45 ACP is historic and is the main caliber of one of America's (and my own) favorite guns - the 1911. That being said, there are also many great modern 45 pistols that take advantage of the latest technology. You also don't have to go far to hear somewhere attesting to how accurate the 45 caliber round is. While 9mm may be faster, 45 does make a bigger hole and when talking about defensive expanding bullets, wider bullet channel generally equates to greater effect on target. While we mention expansion, there is also the concern of overpenetration. 45 (being slower) can lose effectiveness faster when passing through materials, while 9mm (being faster) may penetrate barriers more effectively. In a civilian defensive situation, over-penetration could lead to an unintended target being injured. On the other hand, for large predator defense, there are particularly good loads for 45 (hard cast lead) that can be more effective than what is commonly available in 9mm.
So which is really better? If you want my honest opinion of what I consider the best choice for carry, it's 45 ACP. Why? Because I shoot it better. Anyone who knows what they are talking about will tell you shot placement is more important than caliber, and across the board I generally have smaller groups with most 45 pistols than I do with those in 9mm. However, that isn't everyone experience. So, while I like 45, I recommend you go out and shoot both and make your own decision based on that. Hell, you might find you like .40 S&W, .357 SIG, or anything else if you try them to. What's most important is that whatever you carry, you are confident in - so keep that in mind the next time this discussion (and we all know it will) comes up again.