Most of the people hate the 9mm because of what they've heard from Bob behind the counter or Jim down the street.
I overheard a customer in a gunshop say she preferred the feel of 9mm in Brand X (can't remember what brand X was), but her husband snidely commented something to the effect of "if you shoot a badguy between the eyes with that 9mm you will only make him angry, but if you shoot him with this .45 you will throw him back 10 feet". He left the part out about how if the badguy gets hit in the hand with the .45 it will tear his arm off and throw it back an additional 30 feet
. And if you had used a .44 Magnum, it was have set the hand on fire ... and a .500 would have vaporized the whole arm.
Who knows? Most 9mm loads are, admittedly, mediocre man stoppers, but .38 special is even worse, and I don't see it singled out for nearly as much opprobrium as the 9mm.
<sigh>
How so? The time tested 38+P FBI load will reliably penetrate to the FBI's 12" minimum while also expanding. There are many other 38 loads out there that perform just as well or better. 38 just doesn't look all that great on paper, what with it's typically low kenetic energy. But as others have posted, kenetic energy really doesn't tell us much.
Placement and Penetration are the two most important factors for ammo/gun selection, IMO. Select a cartridge that has good penetration (be it a .38, 9mm, .40, 45 etc) but you can't hit the broad side of a barn with it, then you probably aren't going to be served well. On the other hand, select a load that you can precisely place but lacks penetration and you may also end up in trouble. So if one shoots .45 and 9mm equally well, and ammo cost is not a consideration, then .45 may be the better option. As I indicated, ammo cost does figure in for many of us, and the 9mm has a significant advantage there.
Of course, there are other factors that determine "stopping power". Psycology of the shoot-ee is a BIG one. So is the reliability of the chosen weapon. Confidence figures in too, so if you feel totally helpless with a 9mm, then you should probably pick something else; otherwise you may not perfom as confidently if the crap ever does hit the fan.
I carry .38 and .45 and feel equally undergunned with either - I'd prefer a rifle or shotgun, afterall
. Actually, the nod goes to .38 revolvers by a small margin, as that is what I shoot best and am most confident with. And I carry ammo that gives sufficient penetration, with the added bonus of reliable expansion. YMMV.