Barrel length has little to do with accuracy, except - usually the shorter the barrel, the less harmonics play a role because of the more rigid barrel, thus greater accuracy. There can be feeding problems using .38's as opposed to the longer .357's. If possible, I'd at least try to cycle the action with both using the bullets you're most likely to use. If that is not going to be allowed, I'd get on the website of the manufacturer in question and see if they say the rifle was designed to feed both lengths. I once had a Marlin lever action in .44mag. I tried to feed it .44 specials, but it would not reliably feed them. It seems that my rifle was an older one and not designed to feed the specials. The newer Marlin rifles are designed to feed both.
A .38/.357 rifle will not need a long barrel to achieve maximum velocities due to the relatively low powder volume.
