ORIGINAL: jeepkid
With the .45 you can get a bigger bullet. I believe a 230 gr. where with a .40 the biggest is a 180 gr. I am just going off memory with those two numbers. And .40 S&W ammo is cheaper, and then 9mm is even cheaper.
With the .40 you can get a 200 grain bullet(Double Tap), although there are probably better choices.