At the range today I was approached by an older shooter and our conversation found its way to my opinions on the .40 S&W round. I'm not an expert on the subject by any stretch, but I own firearms chambered for both and have read plenty of research on the two cartridges. And after all that, 9mm wins hands down.
Both of these are Speer GDHP and there isn't too much to say when you compare them side by side. But to me, the difference between them goes beyond ballistics. Availability: 9mm is the most popular pistol caliber in the world. I don't care where you live, if your local store sells handgun ammunition, they will sell 9mm. If you live abroad it's even more prevalent. Cost: 9mm is cheaper to shoot and reload; less brass and less lead. Capacity: 9mm pistols hold more rounds, given a specific pistol frame. Recoil management: this is subjective but I have yet to meet someone who has shot both and thinks the 9mm recoils more than the .40 S&W. Either cartridge can be downloaded for reduced recoil or increased to the point it would be dangerous to shoot, but for factory loads a 9mm will have less recoil.
So lets talk about ballistics. Both of those rounds pictured are going approximately 1150 fps. The 9mm is a 124gr bullet and the .40 S&W is 165gr. Obviously if the .40 is going the same speed but using a heavier bullet, it will have better terminal ballistics, right? Yes, but is that important? A modern 9mm self defense round has nearly equal performance against human tissue, even with clothing. And if it will handle flesh, I should think cardboard to practice on would be no issue. If we are talking about shooting through concrete walls, auto glass, or anything that is "reinforced", yep, I want to be shooting a .40. But lets all be honest. The odds of that happening are nearly zero, even as law enforcement they are nearly zero. If it does come to that, shoot it twice with those extra rounds you have on board and the problem will be solved.
If I wasn't made to carry a .40 S&W at work I wouldn't. It fills a gap between 9mm and .45 ACP, but I don't feel like that gap is big enough to need a filler. I wouldn't want to be shot with either. To me, 9mm is the all around best caliber, regardless of what your end needs are.