I think the 357 Sig cartridge is a great design and a very effective cartridge. In standard sized semi-autos the 125 gn 9mm hollowpoint/softnose gets right out to 1400'/sec, which rivals the older 357 Magnum Police load that boasted of being a 1 shot fight stopper. I am an advocate of bottlenecked cartridges and the Sig cartridge is properly designed to give the best pressure ranges and velocities for the bullets it shoots.
That being said, if I owned a 40 S&W pistol I would get a 357 Sig barrel and probably carry that exclusively. If I purchased a 357 Sig pistol I would not bother with the 40 S&W barrel. But, this is simply my overly opinionated opinion.
If you compare the design of the 357 Sig with the 400 Cor-Bon, you will see that the Sig cartridge is a better design. It allows for more room in the case which I feel equates to more equitable pressure distribution, better accuracy and more consistent velocity ranges than the Cor-Bon cartridge. I once asked the Sig people in Exeter, Vt., if they were going to upgrade their 357 cartridge to a 40 caliber (on a 45 acp case) and they said that althought they would like to, the design would have to be, of necessity, too similar to the Cor-Bon design for their liking. And to be honest, I would put my trust in the Sig people first. Just my opinion. Mikey. though. Mikey.