The 340 is no longer made, it been replaced by the M&P 340, they are basically the same thing. The added weight of a steel or stainless gun makes all the difference in the world in these little J-frames. But then generally, they aren't range or target guns, they are belly guns.
Scandium is marketing for S&W. It's Aluminum with Scandium added. I wouldn't want to pay the premium they are charging for it. It makes the fame a little tougher, but brings the weight down, that's something I wouldn't want, and still not as tough as steel or Stainless steel.
The hammer is personal preference; so the gun is offered either way. The way I carry a J-frame the hammer isn't going to catch on anything, and I may want to kock the gun for an aimed single action shot. But that is my personal preference and why they offer it either way. (Spelling kock the correct way gets it kicked out.
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I have a steel Model 36 I've carried for over 40 years when carrying a small revolver. I had an aluminum hammerless J-frame, for awhile but sold it. It was too light to comfortably shoot at the range, and I want a hammer.
Of those you listed above I would pick the 640 for its weight. But I'd want a hammer, which would make it a Model 60 in stainless .357Mag or a Model 36 in steel .38Spec.
Grab a gun has several of the .38 J-frames in stock in various configurations. Finding anything new in .357 Mag might be harder.
Good luck in your search. If you are like many of us revolver shooters; you will go through many of them. I don't carry one much anymore, but the larger (K-L) frames are my favorite range/target guns.
