The SC models get you another couple ounces off the weight -- essentially allowing you to put Pachmeyer "Gripper" grips on it for no net gain -- these are the nicest fitting grips I have on a pistol -- except for my Dan Wesson. They are HUGE, and they don't put any pressure on the joint of my forefinger (I broke it once, and it's sensitive to pressure.) With that said, I'm a big guy, so I can conceal just about anything IWB, so the increased grip size doesn't bother me. They're also stronger, I don't know if a 642 would have a problem w. a steady diet of +Ps, but I imagine a 340 certainly wouldn't. I do know, according to my gunsmith, that the mechanics of a snubnose can be pretty well wrecked w.o. too much of a blow. I don't know if the extra strength of an Sc frame would help w. that or not.
W. the larger grips, I can shoot an airlight about the same as the steel one w. the "bell" wooden grips -- of course, I would shoot the steel one w. the Pachmeyer grips even better.
Personally, I like my little J-frame, but I carry the pup, and my XD, and, for back woods, at the moment, a Gemini customs SP101, and when it gets done, a Gemini Customs Super Alaskan in 454 casull.