Well, I have thought of this question, as basic as the OP question that is. Not the physics and calculations and all that stuff others have put on here, lol. I liked reading it though, some good info!!!

My personal feeling, is that the 9mm and 38 spl CAN be the same, and it seems that depending on manufacturer and bullet used..... they basically are.... just like I can make a 10mm do what a 40 sw does, or make a 454 Cassull do what a 45 acp does. But..... and to me it's a big but... the platforms are different, and so is the technology, imho.

When 9mm became the wonder gun (glock. High cap pistols, etc) technology and marketing took over, hence alot of R&D. The 38 spl hasn't gotten that kind of attention in many years. BUT, its shot out of a revolver,so the platform lends itself to much more versatility, from cat sneeze loads to wopping 170 grain bullets. 9mm, wont cycle action on cat sneeze and hasnt the space for 170 or higher bullets. So I think if you want versatility to shoot effectively, the 38 spl wins. If you want to stick to 125 grain and 147 subs for suppression, go 9mm, and learn those effectively. I've always thought it ridiculous that sub 9mm isnt that common, especially with suppressors being popular. It's kind of like rifles. Auto vs bolt. Want heavy hitter? Like 300 Win Mag. 338 Lapua? Go bolt, not semi auto. Same principal, unless money is no concern....

In the end, doesnt quite matter with self defense, both are proven loads, both would suck to be shot by, etc. My EDC is a SIG p238, pretty common, but I was jonesing for a 938 (380 vs 9mm). Had a buddy, Green Beret selling his p938. I asked why, he said because he can unload his little 380 faster in a tighter spot. And he told me that means more than a lucky shot with full power. Its advice that other people in that "line of work" will tell you. I never bought a p938, and dont plan to. Again as others echoed. Listen to those that have done it, not the proverbial reload, ballistic processors. Just like people that went to college for something vs the ones that've actually done it for years. Experience is worth a ton in this aspect.

Again. This is just my own observation in this debate. Both are efficient. Just different purposes, like any other debate with guns/ammo.