Well, from an economic standpoint, I just bought some ammo from Ammunition to Go. I got 500 rounds of Federal +P+ 115 gr. JHPs for $141. I also got 250 rounds of .45ACP +P 185 gr. JHP for $144. Both of these rounds have more than adequate stopping power, but one caliber was more than twice the amount of the other. Plus, if you ever have to haul ammo in, say, a bug-out or emergency bag, you'll be able to carry a lot more 9mm rounds than either the .40 or .45ACP.
Finally, seeing that you can carry up to 20 9mm rounds in a pistol vice 8-13 in a .45, I just think it makes more sense to go with 9mm. I do like the steel frame pistols, though. The Smith 5906 is great if you can find one.