This may seem like a harmless question, but it cost me a lot of money a few years back. I had to go out and buy a 4" GP-100 and a P226.
Does the 357 Sig match the 357 Mag? If the 357 Mag is coming from a 4" revolver, yes. And this nonsense about 9mm +++ is just that. The number you're "shooting" for is 1,430 fps with a 124/125 JHP. So, for a defensive pistol that will be carried, more rounds wins. Advantage 357 Sig.
Go to 147s, however, and the Sig is barely keeping up with 158s from the Mag. Longer barrel on the revolver? No contest. Cast 180s? Fuhgetaboutit. Advantage 357 Mag.
Just get both. Or get a 10 mm, which beats both handily. You pays your money and you takes your chances.