All pistols are weak, puny weapons. 9mm is much cheaper and faster to shoot, and is perfectly adequate for stopping an attack when loaded with good, modern JHPs.
If you really want to stop a fight, get a rifle or a shotgun. Otherwise, 9mm is fine. The best thing to do is to settle on one, then move on to the things that actually matter in self-defense: mindset, gunhandling, marksmanship and tactics.