Well "kick" could be more correctly referred to as "felt recoil" (which, of course is subjective, i.e. not all people perceive it the same way). As far as the size of the hole (permanent wound cavity) it puts in someone's body, bullet design (FMJ, JHP, etc.) is going to be a big factor. All things except caliber being equal, among the "service calibers" (9, .40, .45, .357 SIG, etc.) there really won't be a significant difference. Certainly not enough to definitively say that "caliber 'X' will consistently do a better job of dropping someone than caliber 'Y' would." AFAIC, "stopping power" and "knockdown power" are terms that need to disappear from the lexicon of the gun culture.