Here we go again, the age old argument of 9mm vs .45ACP.
To those who say you should not need more than 8 rounds, I say you should not need even 1 round. I'm a believer in Murphy's law and there are many things that could go wrong. You obviously believe in things going wrong, hence you carrying a weapon in the first place. Multiple attackers, low light situations, weapon malfunctions, assailant in body armor behind cover, etc. etc. there is NO penalty for having too much ammo. .
To those who reference 'stopping power' I'd redirect you to a study conducted by the Federal Bureau of Investigation (FBI).
http://www.firearmstactical.com/pdf/fbi-hwfe.pdf
Highlights of the study:
Page 12 paragraph 3
Goddard amply proves the fallacy of 'knock-down power' by calculating the heights (and resultant velocities) from which a one pound weight and a ten pound weight must be dropped to equal the momentum of 9mm and .45ACP projectiles.

Originally Posted by
Cocked _and_Locked
Now there's some scientifically-inaccurate physics-ignorant white-trash logic, if I ever heard it!

According to the same study conducted by the FBI:
Page 12 paragraph 4
A bullet simply cannot knock a man down. If it had the energy to do so, then equal energy would be applied against the shooter and he too would be knocked down. This is simple physics, and has been known for hundreds of years.
In short, I carry a Glock 17 chambered in 9mm, I am more than confident that should I ever need to use it, the weapon would perform equally as well as a .45ACP. Shot placement is key, a .50 is about a useful as a .22lr if you miss. SHOT PLACEMENT is critical as verified by major law enforcement study's.
Quick to the gun, Sure of your grip. Quick to the threat, sure of your shot. - Chris Costa