.357SIG is a nice round, but unlike .40S&W, it's not readily availible if you were to need rounds. The Air Marshall service has all their Sig's chambered in .357SIG for it's personell. Here's part of an article:
Compared to the .40-caliber Smith & Wesson, it is obvious the .40 has much merit. Perhaps there is truth to the rumor the .357 SIG was a marketing ploy designed to lure the last magnum revolver holdouts to the auto-pistol! Cor-Bon's 135-grain JHP moves along in the .40 S&W at 1,325 fps.
A big .400-inch bullet at this velocity has much to recommend it. If you own a .40-caliber, a simple barrel change is all that is needed to make the gun a .357 SIG. So far, springs do not seem needed. The choice between the .357 SIG and one of the hotter .40 loadings is not a simple one. The .357 SIG loads were designed to produce greater vehicle penetration, the Cor-Bon load, designed for personal defense, being an exception.
Here is an auto-pistol cartridge which mocks the magnum, but the verdict is out on its true effectiveness. It probably will never equal the magnum, but seems a good round on its own. My testing seems to indicate the .357 SIG is more accurate than either the .40 S&W or the 9mm. I have a custom barrel in the .40, mostly for the use of lead bullet reloads to which the polygonal rifling of the Glock is not friendly. I have never been able to equal .357 SIG accuracy with this top notch combination. The .357 SIG's accuracy has much to do with the fantastic Bar Sto barrel, but others, using stock pistols, also report good .357 SIG accuracy.
When body armor and heavy vehicles in the hands of gangsters demanded more powerful handguns, it was the .38-44 and then the .357 Magnum that evened the odds. Today, the .357 SIG follows that tradition. It is a good cartridge-a powerful handgun loading which should give good service.
I personally like the .40S&W, but I've been debating buying a glock chambered in .357 Sig.