Well, a pistol bullet can generally only damage tissue it physically touches (unlike a rifle). So if we look at expansion and penetration figures for a couple typical loads, we can get a pretty good idea of what a .357SIG can realistically do versus a good 9mm.
In .357SIG, the Federal 125 gr JHP penetrated 13.7" of ballistic gelatin and expanded to .59". (Reference Todd Louis Green, www.greent.com.)
In 9mm, the Speer 124 gr +P Gold Dot penetrated 13.2" of gelatin and expanded to .62". (Reference Shawn Dodson, www.firearmstactical.com.)
Now, of course, you can play around with loads and find a .357SIG that expands more or penetrates deeper. Ditto 9mm. But the results from these common, typical loads seems to support the idea that the .357SIG isn't necessarily worth the heavier recoil and reduction in mag capacity.
It's an old article, but there's some interesting reading here: http://www.firearmstactical.com/briefs10.htm. Scroll down to find the .357SIG info.