In a self defense snubby, the difference will be negligible and not deter you from practice. I often have carried a .357 4" medium frame gun for a sidearm while hunting. I've taken rabbit for camp meat with 'em. I've been a little bit of a stickler for accuracy in such a gun, but not in a snubby for self defense. My Security six shot 4" at 25 yards off the bench with the best .38 wadcutters. It would put a Speer .357 magnum 140 JHP into 1" off the same sandbags. Also, with that gun, for some odd reason it was WAY off in windage with the .38 vs the .357, only gun I have owned that required more than an elevation change switching ammo. Odd. I bought a M19, would shoot about 2" with .38s, 1.5 with magnums. I had a Rossi 971, 2" groups with .357s, 3" with best .38s. I've sold or traded all three for the use I wanted of 'em, just not enough accuracy with .38 to suit me. Now, I have .38s that do much better, a M10 that cuts just over 1" with wadcutters, but I like having .357s along in a trail gun and the gun serves for trails, also. I traded the Security Six strait up for a Blackhawk that's accurate with .38 or .357, just a sight elevation change, but it's a might big (6.5" gun) and heavy for a trail gun, so I was still dis-satisfied with that option for a hunting/hiking carry. Then, I got my 4" Taurus 66, 1" accurate with either .38 or .357, danged fine trail gun. Now, I'm happy.
So, yeah, long way to say accuracy is usually LESS depending on particular gun with .38 special in a .357 magnum probably due to the bit extra gap to the forcing cone, but it ain't enough to matter in a self defense gun for range practice. I did managed to find one that liked either ammo equally well, though, and I had a SP101 (defensive carry) that was also as accurate with .38 as .357. Funny thing with that gun, shot both loads, hot magnum and light wadcutter, to the same POA. THAT is very unusual. Usually it takes an elevation change, sometimes a BIG elevation change, for the light loads. I have the rear sight elevation screw marked on my Taurus. 6 clicks up for .38, 6 clicks back down for .357. That won't matter if you're point shooting, but using sights, it might put you off paper at 15 yards, something to consider if your gun has fixed sights.
