If the SHTF, over penetration is going to be the least of my worries. .45ACP, because big bullets kill stuff faster.

If you have friends or family, it should be something you at least consider.  Also larger bullet doesn't always mean you kill stuff faster, it could just ice pick through a person if the wrong ammo is used.

I got plenty money for ammo........45 

I'd have both in the event that you don't have ammo for one or the other.  Pretty much everyone else summed up the heavier, lighter, more or less cartridges.  Not certain who can debate "stopping power" since I for one have never had to use my rifle or handgun in either a wartime or self-defense scenario though I have taken a few classes and I know a few trainers of a school whose instructor has been in a few OIS feels that the 45 has better "stopping power."  Surface area, mental toughness, exterior clothing, among others all play factors with the key being shot placement.

http://www.foxnews.com/us/2012/07/28/marines-pay-22m-to-go-back-to-their-old-guns-colt-45-caliber-pistols/

the US Marines ordered 2000 of these Colt M45's for a cost of $22.5 million

Some reports suggest Marines are not happy with their main Beretta M9s for their lack of accuracy and stopping power. With M1911's now supplying Special Ops, growing interest may lead to a better solution.

"To have the 1911 selected again for U. S. Forces 101 years after its initial introduction is just an incredible testament to the timeless design and effectiveness of the Colt 1911," Dinkel said. "This is truly a gratifying contract award."

http://www.foxnews.com/us/2012/07/28/marines-pay-22m-to-go-back-to-their-old-guns-colt-45-caliber-pistols/

the US Marines ordered 2000 of these Colt M45's for a cost of $22.5 million

This 

Plus, I figure if they're so close I have to go to the pistol, I screwed up, or there were just too many of them.

.

the US Marines ordered 2000 of these Colt M45's for a cost of $22.5 million

Seems kind of expensive.

"To have the 1911 selected again for U. S. Forces 101 years after its initial introduction is just an incredible testament to the timeless design and effectiveness of the Colt 1911," Dinkel said. "This is truly a gratifying contract award."

The Colt M1911A1 is probably my favorite handgun overall....

For more than 10 decades the Colt Government Model 1911 has been without challenge the most recognized, most imitated, most influential, and most used semiautomatic handgun in the world. Some call it the best pistol ever. It is to autoloaders what the Colt Single Action Army is to revolvers. If anything, the Model 1911 pistol—in all its forms, chamberings, and variations—is more popular today worldwide than it has been at any time since John Browning applied for the basic patents under-lying its design.  No other firearm on earth can match its record for longevity, versatility, and performance.

Here is my favorite 1911 in my collection...1941 U.S. Army Colt M1911A1. Shipped to Commanding General Springfield Armory, Springfield Massachusetts, 27 Dec 1941. One of my very first handguns I purchased several decades ago and a joy to shoot....

Seems kind of expensive.

Actually the article said they were going to supply 12,000, could be a typo. That's $1,875 per gun, probably including a number of mags and replacement parts.

If cost of ammo, recoil, and availability of ammo are some of the deciding factors, then go with the 9mm. IMO, I don't think that the safety of myself or family should be based on cost, a little less recoil, or availability of ammo for my weapon of choice. In your SHTF scenarios, you should already have been reloading thousands of rounds and be well stocked, because you are not going to be able to find or buy ammo by then. You will have already been shooting the hell out of your weapon to make sure that it and yourself are ready. If any of those scenarios should ever occur, you will know then if you made the right decision.

Actually the article said they were going to supply 12,000, could be a typo. That's $1,875 per gun, probably including a number of mags and replacement parts.

Okay, I figured something like that, I tried to read the article but it kept splitting in two vertically when I'd scroll down, thanks

Meanwhile there's a lot of reports of LEO going back to the 9mm.  Civilians are not subject to the Hague Convention like the military so I think in this instance the LEO trend speaks louder than the military trend since the military is limited in the types of ammo it can shoot.  Of course it could be a case of grass is greener on the other side.  Given the right type of ammo and skill, either have more than enough stopping power IMO.

So if the SHTF right this minute and I had to choose between 9mm or 45 acp I'd choose 9mm (although in reality I'd take my 40 cal) only because I have more 9mm ammo. But the bulk of my 9mm ammo would be FMJ. Is this the case with you guys that chose 9mm also? I have more 45 acp hollow point ammo than 9mm hollow point ammo. What would be better, a lot of 9mm FMJ or a smaller amount of 45 hollow points as far as incapacitating a threat? I'm just saying that if the SHTF right now that's what I have.

This point is not really worth debating as far as efficacy goes.  Pistol caliber weapons are poor choices to begin with.  But if that is all you have, modern ammo options pretty much negates the ballistic debate.  Pick which one you can find, afford and shoot well, then get quality training.  Then if your serious get yourself a long gun for a genuine defensive option and learn how to use it effectively. 

The Colt M1911A1 is probably my favorite handgun overall....
Here is my favorite 1911 in my collection...1941 U.S. Army Colt M1911A1. Shipped to Commanding General Springfield Armory, Springfield Massachusetts, 27 Dec 1941. One of my very first handguns I purchased several decades ago and a joy to shoot....

Damn!

you sure have some really nice firearms!!!