Don't miss my point. The potential lethality of one pistol caliber versus another is irrelevant if you can't hit what you aim. Being able to shoot and hit the gun in the first place outweighs the ballistic superiority of one caliber over another. You can only determine if you are effective by actually shooting it.
Buy the Glock .40 then you can take it down to a 9 mm with a conversion barrel if .40 is too much for you. Or else you will be here debating this until the mods close the thread.