Back when I was really into 357SIG, I emailed Speer with a question on the difference in velocity between them and if there was a "barrier" speed wise, that the 357SIG crossed that made it that much more effective (similar to what you see in the .223's in the AR's) and this is the response I got from them.....
Quote:
The 9mm is a 35,000 psi, +P is
38,500 psi and +P+ is 40,000 psi. The 357 SIG is a 40,000 psi. Bullets
of the same weight will approximate the same velocities in SIG and +P+.
The difference is gun construction, all 9mm's will not handle +P+. All
of the 357 SIG's are made to handle the pressures for the caliber.
Shoot Straight!
Coy Getman
CCI/SPEER Technical Service
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The first part basically tells you that there is really little difference.
As to the last part, interestingly enough, my Glock 31 in 357SIG was beating itself to death, my 9mm Glocks, a couple of which have more +P+ through them than my 31 did 357SIG, show only a minor amount of finish wear at the same point the 31 was tore up. Go figure.
Having shot both 357SIG and +P+ 9mm out of my 31, and comparing them one after the other, the gun handled pretty much exactly the same. By "feel", I seriously doubt you could tell what you were shooting if someone just loaded the gun and handed it to you.
The 357SIG is an interesting round, I just dont think what you might gain, is worth the extra money it commands. I can shoot a lot more for the same money with my 9mm's, and still live in the 357SIG realm with +P+.