I have the original SP101, in 4". It has the full-underlug, and looks identical to its big brother, save but for the smaller caliber.

It is a very cool gun, and is so heavy that it literally has no recoil.
The stock grip was horrible, so I replaced it with a Hogue combat rubber grip. While the gun is fun to shoot, it does have some notable drawbacks. Mine has always shot a little low (even from a rest) and with the staked front sight, it isn't all the easy to just swap it out for one a hair lower, and one doesn't exist, anyway. That's not a deal killer, because once you learn to adjust your aim by a hair, it's fine. Just a little weird.

The bigger issue, and one for which I have never resolved, is after maybe a hundred rounds or so, the cylinder walls will build up with enough residue that it becomes a serious PITA to extract spent cases. Not sure the culprit. I don't know if that is still a problem on the resurrected model? Hopefully, not.

Despite the odd issues with the Ruger, I'll never sell it. It's still fun, and the fact that they don't make that exact model (the new one is really a security six) any longer makes it neat.

I don't own one myself, but I grew up learning to shoot my Dad's Smith 63. I never recall any issues, whatsoever, with that one. I wish Smith would make their current one in a short, or snub barrel. Just for fun. That might bring me to buy a lock gun.