There are plenty of tests to support the point of view that "bigger is better." Gelatin and statisical data is touted all over the place. (Blah, blah, blah type stuff.) However, I have to believe, if you make the first one count the rest doesn't matter as much. The question remains, what gun do you shoot the best?
As a side note. I did watch a video recently of an attempted robbery where the BG took two 45 rounds and staggered away. It didn't knock him down or take him off his feet. He staggered about 10 feet and went down. I don't see that 9mm or 40 is going to matter that much. Perhaps more capacity with the 9mm than the others, but you still have to make solid hits.
As stated above, either caliber is effective with proper shot placement. That being said, I limit all my carry guns to 9mm & .38+p for a couple reasons. 1)I only need to stock two calibers of ammo (in quantity, anyway). 2)These two calibers give me a lot of versatility in my carry options. I can carry my full size S&W if I think I need hi-capacity. I can carry my PF-9 if I really need deep conceal-ability. And if I require a BUG, my Taurus 85 will slip into my pocket. I'm not saying this is right for every one. it just seems right for me.
Go with what YOU can handel without any problems. I have carried some type of handgun for over 45 year's and still do but now I carry a .380 Walther with Speer Gold Dot Hollow Points and I am confident in my ability to take out any threat that should come up and so is my wife with her Walther .380. I carry the Walther because of Medical reasons and that I can't take the higher recoil of either the 9 M.M. or the .40 Cal. with my pain sensitive hands. Altho I own weapons of both calibers and will shoot them from time to time.
I have both 9mm and .40 s&w. I like both calibers and cannot tell a lot of difference in recoil. Like others have said, the 9mm is cheaper and probably more readily available.
I shoot a Glock 19 and a SR 40 C. They are both sweet shooters. I do use the 9mm more, due to the price, but I feel either is a good choice. Still you need to shoot both and make your own decision as to which you prefer.
I also have a Walther PPK 380. It is the old styje and was wondering was yours the same. They call it the "James Bond Gun"
No they are not alike at all. The PK380 is a little bit bigger and has an all polymer frame and a much better hand fit. Bigger sights and fully ambidextrious for both the mag. release and the operation of the safety. It is also a 9 shot weapon.
I traded in the Walther PPK/S .380 that I had gotten for my wife and got her a PK380 a few years ago. Yes I took a loss on the trade but as she is more then happy with the PK380 I don't care.
The only problem that I have with this weapon is that it only comes with one mag. and I had to put out almost $80.00 too get her and myself each a spare mag.