Either one is fine. Our agency is now transitioning back to 9mm from the the 40 cal. The reasoning is not effectiveness of one over the other. The reasoning is more rounds in the gun and more hits due to a slightly less recoil impulse. Last, Glock offered a crazy deal on the cost of the switch over. We are carrying the the Federal 40 cal. 180 gr HST and will be carrying the Federal 9mm HST 147gr. Absent a CSN shot and instant drop, out of the fight result ,using a standard pistol round to stop a threat is like using a small hammer to drive a big nail. It can be done but, the results are not always ideal and vary widely. People often do not know they are hit and the effect of the hits can take awhile to be observable. some people are dead on the their feet but do not know they are dead. Hence the need to keep reevaluating that threat and reengaging or disengaging as needed.