The "Job" your gun will most likely do is provide you with hours of entertainment, and building your skill with a firearm.
Pick the round that allows you to do that, and forget thinking about which fictional hypothetical badguy you will likely never meet would go down to a .40 when he wouldnt do a 9.
That is my advice.
Chance of life being threatened. 1 in 100
Chance of life being threatened and situation requires the use of deadly force. 1 in 1000
Chance of life being threatened, deadly force required, criminal shot with a 9mm and you DIE because he would have been stopped with a .40...
What? 1 in 100,000?
Play the lottery before worrying about those odds.
This is a perfectly good response. You should go with it. I agree totally.
Unless, of course you want to impress your gun enthusiast buddies... then you must have a .40 (sorry, bigger is better).