W-231/HP-38 and Unique will do similarly in .357 Magnum. They will give you nice medium power loads. I am not knocking Unique, but to think you need to ditch 231 and get Unique is six in one hand, half a dozen in the other. You also don't need mag primers with either of those powders.Have you used HP38 with .357 magnum loads? I have HP38 on hand, but am getting ready to load some .357 magnums, and wondered if I should get some Unique, and some magnum primers, to do the .357's.
If you want full .357 Magnum power a slow powder like AA #9, H-110/W-296, Enforcer, N-110, 2400, etc, etc will get you more power with a less snappy recoil.
W-296/H-110 needs mag primers. AA #9 seems to be fine without them, as is 2400. N-110 doesn't seem to need mag primers., but I haven't tested it fully. I use 2400 for full power .357 Mag, and even then, I am not at the ragged edge of max. 14.5 Grs of 2400 and a coated 158 Gr SWC is powerful, accurate, and fun. If I want more than that, I drag out the .44 Mag.