Great question? 9mm moves at about 1000 fps. 40 caliber moves anywhere from 1250-1500 fps. 40 caliber has the stopping power that the 9mm does not. I personally use 40 caliber Federal Hydra shock hollow points. What about recoil difference of recoil? 40 caliber has a bit more recoil than 9mm, it depends your size, strength. If I am forced to shoot someone to defend my life, with 40 caliber I know the damage it will cause them and the quick amount of blood loss, they will be going down quickly. Then again a well placed shot will do its job. If a person is on meth, pcp they will probably continue to come at you. Thats why we strongly recommend at least 3 shots quick into them, if they don’t go down Im shooting straight to the head. In finishing finishing I choose the 40mcaliber simply because of the stopping power it has, but not everyone is capable of controlling the recoil of a 40 caliber.