The 9mm Luger FMJ has a reputation as an outstanding penetrator, better than 45 ACP FMJ. On top of that, according to (brace yourself) Ed Sanow the 9mm Luger FMJ always tumbles 180 degrees in gelatin, due to the shape of the bullet. So...if I were stuck with FMJ I would be just as comfortable with the 9mm as the 45 ACP. The 45 generates a marginally wider hole while the 9mm ensures better penetration and is more likely to tumble.