It really is personal choice now. The ammo manufacturers are making very good 9mm SD rounds. This is why I decided to make my EDC 9mm. I am accurate with this round and have confidence in the gun, ammo and my markmanship.
Cost factor is really nice for the 9mm as you can buy FMJ for 8.97 per 50 at Walmart.
I do not believe you can make a wrong decision between the 9 or the .40
That is my .02 worth.
Once you buy it you must post pics!