I too am looking for the right load. If I had wanted a 38 Special, I would have bought one, so stepping down from 357 is not on the table.
Ruger SP101 (3+" barrel) is a lightweight frame with 5 rounds, so heavier loads are pretty punishing. First time out with Federal 158 Gr 357 gave me a really sore knuckle behind the trigger guard. Number one, I will change grip accordingly but have ordered the Hogue rubber grip. I believe I need a milder round but based upon a different weight bullet. I bought 5-6 different weights of 38 cal XTP, but the testimony about use of 125 grain matches my intuition.
I also bought H110, IMR4227, and 2400 as powder choices that I believe will allow good volume within the case, consistent detonation, and fast enough burning to preclude excessive flash and suboptimal velocity.
This is just background, not a statement of position, because I would welcome advice or affirmation regarding my choice of direction here.
p.s. I don't have any small magnum primers and cannot find any, so assume small pistol primers here. My supply is all CCI.