Retired LEO here with 38 years OTJ and 4 years as a USN Aviation Ordnanceman ('68-'72).
My suggestion: As a number of public ranges that sell firearms also "rent" them out for range time, or allow you to shoot one free for nothing more than the cost of ammo they sell (in the hopes that you'll buy from them), take time to fire both, the 9mm and the .40, then decide which one feels most comfortable, and provides the greatest degree of control and accuracy.
Hitting your target with one or two well placed rounds always beats the "Spray and Pray" method. Good luck and let us know what you decided.
My suggestion: As a number of public ranges that sell firearms also "rent" them out for range time, or allow you to shoot one free for nothing more than the cost of ammo they sell (in the hopes that you'll buy from them), take time to fire both, the 9mm and the .40, then decide which one feels most comfortable, and provides the greatest degree of control and accuracy.
Hitting your target with one or two well placed rounds always beats the "Spray and Pray" method. Good luck and let us know what you decided.