I took my SP101 and S&W 640 along with a bunch of different 357 ammo to the range yesterday. After testing for POI to the POA with the fixed sights at 15 yards, it looks like both guns favored 158gr over 125gr. All the 125gr loads shot about 2-3" low at this distance. (It's a shame. The 125s had noticeably less recoil.)
Both guns liked Remington 158gr HTP, so it looks like that's what I'll be using.
Lucky Gunner gel testing shows this round as a good performer out of a snubby.
Federal 158gr Hydra-Shok was a close runner up, with the 640 grouping slightly better than the SP101 with this round.
Scubaoz gel test showed this to be a nasty round out of a snubby as well.
Interestingly, Winchester 148gr 38 Special Wadcutters keyholed out of the SP101, though the oblong and torn holes were at POA. (I brought those for some relief on my wrists and to check for flinch after shooting back to back magnums!) These rounds shot beautifully out of the 640.