All depends on the shooter. I am not recoil sensitive (shoot a 44 Mag handgunfor deer hunting). However, there is a noticeable difference between a 9MM and a 40S&W or 45ACP. That being said, if this is going to be used for self defense, I would NOT go with a 9MM in the 24/7. I do NOT belive this is approved for +P ammo in 9MM, so I would want a 40 or 45ACP (stay away from 45GAP - higher $ ammo and harder to find). IMO, the recoil of the 40 has more of a snap, and the recoil of the 45ACP has more of a push.