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Some people maintain that 38's shot in a 357 are less accurate because when the bullet leaves the case is travels through a little more "freebore" in the cylinder before engaging the forcing cone. I have no idea if this is true or not, nor if true, how significant it is. But from my own experience with both 38 cal. guns and 357 cal. guns, the effect seems to be quite small.

I hold an expert rating in NRA bullseye, and I am no where near a good enough shot to discerne any difference.

One "advantage" of a 357 cal. gun is that you can be sure it will last almost forever shooting the much lighter 38 spl. loads.