skipping the "which round is more killy" and the ballistics info, in the end, with quality ammo, you cant go wrong with any of them. I prefer 9mm for the obvious reasons that I can cram more rounds in a magazine, its cost effective, and it is generally more controllable. Its all preference. There is no right answer. For me, .40 is to snappy in most firearms and .45 is great but you typically give up mag capacity. Being a civilian, I dont expect to get into an intense gun fight where a large number of rounds are necessary but it doesnt hurt to have that option. They all work. There is no magic round that will drop a BG where he stands. If youre asking for an opinion, I say go with 9mm. If your asking for a right or wrong answer, quit asking because there isnt one.