My experience is that I simply shoot better with a 9mm than I do with .40
The recoil is a bit too snappy with a .40 for my taste, and with the 9mm I can shoot more rapidly because it's easier to re-acquire my sight picture after each shot.
With some of the advances in ammo, I really don't think it makes a significant difference which one you use. Shot placement is far important.
Thanks for considering the use of paragraphs.
And... there's a lot of .380 guns out there, so one would think the ammo would be cheaper, eh? Ahh, the mysteries of capitalism.