chinto
Senior Master
True, that bigger can be better, if we're talking about ball ammo. After all, a .357 magnum will make a hole about 0.357" wide, and will probably make an exit hole of similar size, while a .45 ACP will do the same with a 0.451" diameter hole.
However, with today's jacketed hollowpoint ammunition, the playing field has been equalized. Take a look at Winchetser's ballistic's:
http://winchester.com/lawenforcement/testing/testing_pg.aspx
Not much difference at all between a 9 mm and a .40.
Also, look at the Ranger T series:
http://winchester.com/lawenforcement/testing/testing_t.aspx
Not much of a difference between 9 mm / .40 / .45 ACP.
Similar premium ammunition from any of the major manufacturers (Remington, Speer, Federal, Hornady) should perform similarly, too.
actually a jacketed hollow point from a 9mm and from a .45 cal will tend to expand fairly closely in percentage of increase in diameter. but for self defense i would suggest you use a frangible ball like "mag safe" or similar... prevents over penetration and other problems that result in secondary casualty and collateral wounds to innocent bystanders. but the larger heavier slug will be more effective in most hit locations at incapacitating the attacker faster, and more effectively.