Very good comments by everyone on the subject. As always, it boils down to "personal preferance" based on what the needs are.
Many shooters beleive that the 9mm is somewhat "anemic" for self defense and that bigger bullets have better stopping power. There have been lots of new bullet shapes and powder loads developed in the last ten years or so for all calibers available. Stopping power has become even more arguable now.
The .40 is viewed as an excellent compromise between the 9mm and .45ACP. Less recoil than the .45 while maintaning bullet mass and a larger magazine capacity.
Just figure out what YOUR priorities are and go from there.
Points to ponder:
1. Ammo cost and availability; 9mm is plentiful and inexpensive.
2. Magazine capacity.
3. Initial cost of the handgun. May not be an issue since most manufactures make both calibers for the same model at the same price.
4. Do you hand load your own ammo?
5. Defensive shooting or just target/plinking.
6. "Taste great...less filling..." you be the judge.
Sneaky jms suggests you try a .40 S&W mag as well if you go 9mm. Many 10 rd. .40s will hold and operate with 15 rds. of 9mm. Only for emergencies, but if it works.....hehehe. Bottom line, the .40 has a bit of knockdown power and is more suitable for IPSC matches, steel plates, bowling pins, etc. Factory 9mm can often be found for $6 a box, sometimes less. If you just want to go out and shoot at paper or plink, the nine is fine. If you have real defensive concerns or want to do the organized shooting games, go .40 S&W.
Nice find, mike. Thanks. A question for the Gun Experts here. Why would they make a semi-auto pistol to chamber .357? Aren't .357's usually a wheel-gun load? Is there an advantage to that caliber? Would YOU want one?
Also, they say they're shipping M&P9s, but they don't say how many or to whom.
A .357 SIG is a .40 case necked down to take a .357 bullet, 9mm more or less, (someone with more experise will probalby tell us it's really .358 or whatever). The extra powder gets you higher velocity.