One is clearly more powerful than the other.
Well, sorta, on paper anyway. Going strictly by the numbers on paper, one has "a bit" more power, but its really nothing major. But, if youre going strictly by the numbers, the 357SIG does win in the math department.
Let's also not forget that the .357 Sig is built for that kind of pressure.
While I agree, thats true for most of them, but I think what's being missed here is, with 9mm, you dont have to practice with +P+ to remain proficient. There is really little difference in how the gun shoots with standard 9mm, and +P+ (or 357SIG for that matter). We arent comparing the difference in .38 wadcutters vs 357mag full power loads here.
I had a Glock 31, and I have a number of 17's. My one 17 has at this point probably double plus the number of rounds of +P+ 9mm through it than my 31 had 357SIG through it, yet my 17 only shows some minor finish wear on the underside of the slide, where the 31 was battering itself to death shooting 357SIG. So there are exceptions to the "made to handle" thing. The 31 never showed signs of stopping the battering, where the 17 is still chugging along without issue, and after 140000 rounds of standard, +P, and +P+ 9mm, is still only polishing the finish on those two spots on the slide.
As I said before, Ive been on both sides of this little argument, and while I like the 357SIG, I personally just dont see the "math", here, and from all angles, not just the paper velocity numbers.
If you like 357SIG, Im sure it wont let you down, if you can afford the ammo these days, and youre well practiced with it. Truth be told, if you buy the same model gun in 9mm, and use it in practice, youre going to be ahead, both in skills and money, and I seriously doubt you will notice any difference in shooting them.
Or you can keep it simple, and just use 9mm guns, and keep +P+ in your carry gun.