If you are in free state that allows you to have standard capacity magazines, do as you want.
However if you are in a limited capacity state, stick with the .40 or upgrade to 10mm
The .40 S&W when used with top of the line high energy ammunition has a significantly greater penetrating ability as well as energy delivery ability than the 9mm. (Based on our actual testing), especially with hard barriers. (Now .357 Sig is also very interesting in that regards).
The reason everyone is going back to 9mm love is, cost, size, capacity and ease of shooting long strings.
The .40 is going to cost a bit more, you are going to have less capacity and it is not nearly as easy to shoot well, especially for older/weaker/female/small gun carrying folks.
What matters most is that you have a gun you can shoot well, practice with lots and carry all the time.