I believe Mr. Sinko has given the most cogent case against the .38 Super in a .357/.38.

If nothing else, optimally the .38 Super takes a jacketed .355 bullet while the .357 does take a jacketed .357.

Given that S&W has usually been generous in cutting its barrels, unlike a Colt Python that cuts them around .355, the Smith barrels with .357/.358/.359 dimensions just doesn't allow
for the best accuracy with the .38 Super's .355.

As an aside, I don't believe the 9mm is all that great shakes in the Ruger Blackhawk .357 convertibles when it comes to accuracy.