The whole point of the .357 SIG was to duplicate the older .357 magnum 125 grain JHP load's excellent street performance, performance that many missed when the switch to 9mm autoloaders came around as mainstream. Right?
I myself consider both the 9mm and .357 SIG, with today's modern bullet tech, to be both good for what they are. I like my Glock 19 and it's 9mm round just fine, there are many good loads for it now.
The way I look at it the .357 SIG obviously is a more energetic round than the 9mm. And this energy can be used to do more "work" like penetrating a very large person or a car door. But, the 9mm generally can do enough "work" with it's energy level for most self defense situations. I feel fine with a 9, but would never fault someone for carrying a .357 SIG. It to me is just a 9mm that can do more "work" if someone needs it to in certain situations.