Very interesting thread, but I have found some contradicting data from Hodgdon's online data manual. Most folks here seem to agree that a slight bump in powder weight using 357 brass is beneficial, however Hodgdon appears to claim the opposite! I used their calculator and selected 148gr HBWC & HP-38 for both calibers, their results:
38 SPECIAL
3.5-4.0 grains, 869-956 fps
357 MAGNUM
3.0-3.4 grains, 845-908 fps

Note the MAX load for 357 is BELOW the 38 STARTING load ??????????
Very confusing to say the least! I think I've lost my faith in Hodgdon's calculator. Any thoughts?