Either one will serve you well G19 or G23. If you are really concerned about ammo prices then the 9mm is the way to go. If you want a tad bit more power then .40 is the right choice. WWB 100 pack 9mm is about $15 for me locally while the same value pack in .40 is about $20. Not much of a real difference considering that the same value pack in .45 is around $28-$30. I used to own the 357SIG caliber, but at $20 for a box of 50-count ball ammo, it had to go. I say just flip a coin and decide, I own both and love both.
P.S - I also went through a consolidation lately pertaining to my having too many calibers. 9mm is
great, but in my opinion, in order to feel fully confident with it, you have to choose some sort
of +p, of +p+ ammo, in a gold dot, or winchester ranger configuration. In .40 just get a good
hollowpoint round and go with it.