People put far too much emphasis on the loading, and not enough on the gun. First-time shooters want "a .40" because that's what their friend told them they needed, based on something they heard from a guy who once knew a cop. There are usually mentions of stopping power and/or knockdown power.
.40 is a high-pressure loading in a short casing, and it's darned snappy. It is not a beginner's cartridge any more than .357 Magnum is, and it's no fun at all in the small, light guns that are so much in favor these days.
I'll take .45 in a steel gun over a Glock 27 any day of the week, and as Kathy pointed out, new shooters place themselves at a real disadvantage by choosing the latter.
...of course, if I had my way, everyone would start with a .22, then work their way up to .38 in a medium-frame steel revolver before ever touching an automatic in a service cartridge.
In the depth of winter I finally learned that there was in me an invincible summer.