9mm +P is just the designation that says that's what the load is, the cases are no different.
IN .38spl, I don't know if there is a difference, I don't load +P 38 since my guns are old. I separate and sell the +P headstamp 38 cases.
The basic SAAMI spec for pressure is the base pressure. For 9mm Luger, it is 35,000 psi. 9mm Luger +p is 38,500
.38 Special is 17,000, .38 SPl +P is 18,500.
Bear in mind that both cartridges have had their limits reduced significantly in recent years from what they once were.
If you are just shooting targets or practicing, +P is not even really necessary, paper targets won't know the difference. If you need +P to duplicate a self-defense ammo, that's a different story.