The .357 Sig does not match the .357 magnum ballistics, that is an urban legend. It does not perform better from a defensive stand point than +P 9mm rounds and is more expensive to shoot. It's only advantages are it shoots flatter at distance, but minimally, it's louder and harder to find ammo for.

What you essentially have is a 9mm bullet sitting on top of a .40 body. If you feel compelled to buy a .357 Sig, buy a 10mm, then at least handloads provide some of the advantages claimed for the .357 Sig.

Everybody is different, but there is no distinct advantage to the .357 Sig.