There are many experts that will tell you that the 9mm is somehow not up to the task of self defense. I suggest you do your own testing along with reading the specifications for stopping power online and checking the gelatin penetration videos.
A friend of mine tested his .22 Ruger LCR pistol by firing it into a phone book. It went 600 pages into it so this convinced him that a hole at least that deep in a human body would likely dissuade many attackers unless they are amped up on drugs.
He took his Makarov and tried the same test. It went through the book completely and traveled onward. This simple test gave him a high level of confidence in his weapon of choice.