As in everything, shot placement and ammo configuration is everything. The military is screaming that they need a return to the 45 ACP. That's because they HAVE to use ball ammo according to the rules of land warfare established by the Hague Convention (IIRC). All things being equal, a 45 ball load is going to be much more effective than any 9mm ball load. However, switch to state of the art JHPs and the differences drop dramatically.
I'm not certain what bad advice has been given on this thread. Most folks seem to have well reasoned and experienced suggestions each coming from his own unique background and level of training and experience.