The wide variation of .357 Magnum loads and the effect of cylinder gap kind of makes it impossible to state definitively.
I think the velocity gain from adding barrel length peaks for most loads around 4" to 6". That is, you gain a lot of velocity from barrels up to that length, but after that length the gain is much smaller, to the point that if you have a 20" rifle, you're probably not gaining more than 5 fps or so and may actually be losing velocity over a 16" or 18" carbine.
Other loads are optimized for short-barrel snubnose revolvers.
My personal preference is 3", which is a compromise between the full effectiveness of 4" barrels and the size of the 2", while retaining most of the velocity gains from the longer barrel and minimizing the muzzle flash and flip of the snubnose.