But, as others have noted, I have detected a subtle, latent wish from the op to be talked into getting a .45. If this is the case, get one! There's really no "wrong" choice with either cartridge and you can always get the other one later.
Perhaps that is the case.
My last issued "duty gun" was chambered in .40 S&W. It was a great match for the gun. No "snappy" issues. As the bullet flies, I have two identical guns in Officer Size, one being a .45ACP and the other in .40S&W. I can shoot the .45 ACP all day long, the .40 tends to hurt my hand after a couple of hours on the range. It might be noted that the frame on these guns was light weight aluminum composite while the slides were stainless.
So to sum it up, both rounds are good defensive rounds but if you are looking for a bit more mass in your bullet and a longer staying power at the range, go the 1911 route. I never met a 1911 I didn't like.