A friend of mine is a ballistics scientist with the VSP forensics lab, here in Richmond. He tells me that over-penetration has not been a problem with 357 SIG in shootings he has researched (probably by Richmond PD and VSP) over the past decade, where the JHP's expanded as expected. There have been some bullets retrieved from the clothing on the far side of the body, indicating complete penetration of the body, but not enough residual energy to penetrate the clothing layers on the far side.
I have a .40 conversion barrel for my P226, but prefer 357 SIG. There may not be a lot of variety in ammo, but the 125gr 1450fps pairing in a modern 9mm JHP is difficult to beat. It seems to be a sweet spot in ballistics. 115gr, 147gr, sub-sonic and exotic loadings are available but the 125gr appears to be optimal. The round shoots flat and hard, only dropping about 3" at 100 yds and still moving at about 1050fps. It is a remarkably consistent cartridge, with an average variance of less than 20fps in the Gold Dots and RA357SIGT's. It is inherently more accurate than the .40, but for combat shooting, the practical difference is negligible. I have never had a failure to feed, failure to eject, squib or misfire in 357 SIG. I cannot say that about any other cartridge.
Internet prices for bulk 357 SIG ammo are becoming competitive with premium ammo in other calibers. It is somewhat more expensive, but I am getting +P+ performance from a SAAMI spec standard round, so you really have to compare standard 357 SIG to +P+ 9mm to get an accurate comparison for the cost-benefit of the cartridge. Reloading dies and supplies are readily available.
Flash and noise are always a point of argument with 357 SIG. A 357 SIG load is the equivalent of a 9mm +P++. If you shoot a hot 9mm next to a 357 SIG, they will sound about the same. If you shoot a hot .40 next to a 357 SIG, they will sound about the same. If you think about it, this makes sense. Rounds of similar performance sound about the same; compare a 357 SIG to a standard 9mm, and the 357 SIG will be a lot louder - that makes sense, too. Flash depends most on the cartridge. I shoot Gold Dots or Rangers, both of which use low flash, very clean burning propellants. I don't have problems with losing sight of my targets in low-light conditions, and the flash seems about the same as 9mm +P+ or .40 +P ammo, at least to me.
I chose 357 SIG for a variety of reasons after careful consideration of why and how I was going to carry. That doesn't make it the right choice for anyone else. For me, it seemed to be the best all-purpose defensive round. Some will disagree. I am confident of my choice. If 357 SIG were not available, I am equally comfortable that a premium .40 would do the job, too.