1) 9mm weapons almost always carry more rounds then their .40 cal partners in the same size frame. I say almost always because where as I couldn't find one that didn't, I far from know everything. So I would get the benefit of 1-3 rounds more, in the same size package, and I can't think of a situation where I would ever complain about having too much ammo.
2)9mm is cheaper, both for plinking/training, as well as carry ammo. The benefit being obvious, it will allow me to spend the same amount yet shoot more. But could that lead to shooting more just for the sake of shooting, instead of training more?
3)The few times I've shot 9mm, (and its been years), I seem to be able to shoot faster follow up shots then with .40. I have no data to back that up, it just seemed that way, and I guess it makes sense, since 9mm recoil less, but again, I don't have any proof of that, and it could just be gun shop dogma sneaking up on me.
4) I found the chart below while I was looking around online. I know it's from a manufacturer and not an independent, But I figured as long as it was comparing it's own products to each other, it would be alright. But comparing the heaviest 9mm to the heaviest .40 cal, I really don't see that much of a difference. The 9mm penetrates gelitin 10", expanded to 0.87 inches, and gets there at 1260 FPS, while the .40 cal penetrates 12", expanded to .96, and gets there at 1041 FPS. So all and all, we are talking about two more inches of penetration, .09 inches of expansion, and a 219 FPS speed difference. Am I wrong for thinking that those numbers are just to similar to really make a difference?
I know that I won't necessarily be under gunned if I choose the 9mm over the .40cal, But I really am hoping someone a little smarter then I can tell me either I am on the right track as far as knowledge and comparing them in a intelligent manner, or at least show me where I got off track
Thanks for reading through my long winded rant, and I hope it made sense to someone.