Which is better, 9mm vs 40s&w?
9mm for practice, competition and beginners. 40sw for real work. They hit as hard as 45 but you can carry more in a smaller platform. There WILL be recoil!!!
The .40 Smith and Wesson has more stopping power than the 9 mm Parabellum, but in most realistic situations the average shooter is not going to see a lot of difference in performance. The 9mm is going to cost less to shoot, but if a person were looking for a sidearm to carry as a bit of insurance against a bear attack in black bear country, the .40 S & W would be the better option. As far as personal defense is concerned, it's like comparing one apple to another.
This sort of question comes up on here a lot. I almost always say 9mm. Modern handguns and ammo perform very well. Lots of different statistical looks at shootings indicate that on average you're still going to need more than 2 rounds to incapacitate an aggressor, a one-shot-stop only occurs about half the time. 9mm ammo is particularly cheap - for the same type of ammo, 9mm is almost always going to be cheaper than other handgun ammo. The more affordable the ammo, the more you can (and probably will!) practice. The more you practice, the better you'll be at shooting. And for any 2 guns of the same make, the 9mm almost always holds more rounds. So if the 2 rounds are giving you the same performance benefits in a self defense situation, the 9mm gives you more rounds in the gun and more opportunity to practice.
Which ever one holds the most rounds- period. If you ever get into a shoot out the- first lesson you will learn is that you can't hit the target. Shooting at a still paper target at the range is one thing. Shooting at an aggressor that is moving and shooting back at you is a completely (ed. "different") story.
better for what
9mm is cheaper and easier to find. Both have significant power.
It appears that 9mm is becoming the ideal round and it has been around many years. I would use either depending what I had available. I have both and I like and use both. Different people like different things so it is hard to answer. Whatever you choose will be a good choice.
For what purpose?
40. 9mm is at best good for a backup pistol.
9mm and 40S&W are not dangerous and will never be banned. Both are equally deadly with proper shot placement. Both are equally ineffective when you miss. One has less recoil than the other and can cram more boolits into a similar sized magazine.
9. No contest. .40 is a notoriously unreliable caliber.