I've used a chronograph for about 20 years now and the powder makes a difference with a .357 Mag in a snub nose.

The revolver itself also makes a difference, as bullet placement matters, and shooting .357 Mag in a small revolver you can't control is a poor choice that will get you less than optimum results.

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Contrary to popular internet rumor, the slow burning colloidal ball powders that produce maximum velocity in a 6" barrel, will not produce maximum velocity in a 2 1/2" or 3" barrel. Those heavier charges of slow burning powder will however produce a lot more recoil, so shooters think they are getting more velocity, when they are often getting 100-150 fps less velocity.

For example a max load of Win 296 and a 125 gr bullet will give me 1,132 fps, while a maximum load of Unique will give me a velocity of 1,296 fps with the same bullet in the same 3" revolver. That's a loss of 161 fps with the slower burning powder in a short barrel.

In comparison, a .38 +P load using IMR 800X with the same 125 gr bullet will give me 1,030 fps in the same revolver. That's a loss of 102 fps compared to the slow burning powder .357 Mag, and 266 fps compared to a .357 Mag load optimized for a short barrel.

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How much difference does 1,030 fps, 1,132 fps or 1,296 fps make? It really depends on the bullet. Provided it's operating within it's performance envelop you should get adequate penetration and expansion.

That's almost certain with the .357 Mag, given that there are not many poor performing .357 Mag loads, even in a snub nose revolver. However, you need to be a little more selective with .38 +P loads in a snub nose.

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I usually carry .357 Mag in my 3" Model 13, 2 1/2" Model 66 or 1/2" Model 686+ revolvers because they are comfortable enough to shoot with full power loads that I will practice with them enough to become very good with them.

It's worth noting here that I use a load that is optimized for a short barrel not just for the improved velocity, but also due to the reduced recoil from a powder charge that weighs half as much.

However, with my 3" Model 60, I'll carry the above mentioned .38 +P load as it offers adequate terminal performance while still having comfortable enough recoil to practice with it extensively. In contrast, .357 Mag loads in a J Magnum frame revolver, even a steel framed J Magnum revolver, are not comfortable enough to shoot extensively, and thus you won't shoot it enough to become truly proficient with it.