#1 9 times out of 10, the attacker seeing the victim is armed with ANY firearm, is going to turn tail and flee
#2 getting shot once by anything will often take the fight right out of the attacker, causing him to drop down, surrender, curl in a ball, scream and cry, whatever, be it a scratch or worse
#3 shot placement seems to ultimately be most important, a hit to the head or to the heart by even mediocre rounds is going to cause fight stopping damage.
That being said, there are some differences between the two rounds. Because we can't shoot prisoners to test ammo, and because of #2 and #3, it is hard to tell exactly how big the differences is. But take a look back at history, at one time people were carrying 32 short and 32 long pocket revolvers, 32 acp was a common chambering for european police forces to use. Now, I am sure that no 32 acps from Walther PPKs were bouncing off anyone's skin, but I also know that a 45 acp is going to do a lot more damage.
Also looking back at history, both military and civilian/lawenforcment you will see times when it has turned out that certain calibers were far from sufficeint, times when people took multiple 38 special rounds in the torso and kept on fighitng. Of course, for some of them, we'll never know if a 45acp would have ended the fight immediately or if he would have take 5 45acps rather than 6 38 specials to stop him.
But, lets say the difference is something on the order of 9mm has a 80% chance of a one shot stop, 40 SW has an 85% chance, and 45 acp has a 90% chance, all with quality hollowpoints and center mass hits. how much is that 5 or 10% increase worht when your life is on the line.
Also, I have never heard anyone say he 357 sig is BAD. The truth just happens that while it is supposed to dupliate 357 magnum ballistics, it tends to fall a bit shy of the true 357 magnum with lighter end bullets like 110 grain and 125 grain. Add to that the fact that hte 357 sig isn't anywhere close to the performance of a 357 magnum with heavier 140 or 158 grain or 180 grain hunting bullets, and that there is no 38 sig-special for cheap and light recoil practice, most people object to the claim that the 357 sig is 'as powerful as a 357 magnum, but you get twice as many shots!" most people will heartly agree that the 357 sig is better than a 9mm, although most people will also say that there is no reason to neck down the cartridge of a handgun, give em the biggest bullet the case can hold, so the 40SW will always be a little bit better than the 357 sig.
heck, I LOVE the 357 magnum, but if I could just wave a magic wand and make it shoot even bigger bullets with no increase in gun size or recoil or anything, you bet I'd do it.