I look at it from the standpoint that each has proven itself to be an effective pistol caliber. The 9 and the 45 have very long established records while the 40 is still relatively new.
Pick whichever one you want for whatever reason you want, no mater which you pick, you picked a good one. Then practice, practice, and practice some more
In my case, economics has helped make the decision for me between 9 and 40. I can buy, and therefore shoot, a lot more 9mm then 40cal, so I tend to gravitate to the 9. With modern day loads and bullet designs 9mm is a good performer, and in general you can carry more in the weapon, and more on your belt. For those reasons the G19 tends to get used a lot and the G23 spends most of it's time in the safe.
Oh, and for what it’s worth, the 45 is the death ray
