The main advantage to the .357 Sig is that it basically has the same stopping power as a .357 mag in a much smaller package. The .357 mag is widely recognized as being one of the most effective "one shot" self-defense cartridges in existence. The Sig round is a very high-pressure round though, and is known for being hard on the gun itself. I suspect a lot of this could be alleviated through alternative spring selection, but I'm not sure.
cosmoline is an aphrodisiac!