There is no doubt that even in short, snubby barrels the .357 Magnum will generate more velocity than a .38 Special, +P or standard pressure, given the same bullet weight.

However, as has been pointed out above, in the real world the service calibers all perform about the same. So, taking "stopping power" off the table, what we are left with is that the .357 Magnum will generate more recoil, muzzle blast, and, depending on the load, muzzle flash.

To me, whatever velocity gains the .357 Magnum has is overshadowed by the higher difficulty in getting quick hits on target. For self defense, I don't see a reason to go magnum when .38 Special +P will do the job with better shootability.

But, if you prefer the .357 Magnum in a snub, it's reliable in your gun, and you can shoot it well, there's nothing wrong with doing so.

Just my opinion.