Interested in a J-frame...
From what I can figure out, these are the key advantages in content that a 340 has over a 442/642:
Blast shield on top strap - This is probably the single biggest deal for me, as without it, the frame is being eaten away. I wouldn't put a high volume of rounds through it, but having a scandium frame without it would bug me. Can anyone say whether the blast shield is the feature that makes it ".357 capable", or is there something structural about the 340 that's different?
Protected ejector rod - Obviously, lots of people carry the 442/642 without bending an ejector rod, but it would make me feel that much better knowing there's insurance here against a possible damage mode that can severely impede function.
Pinned front sight - Can be swapped out if desired without involving somebody who owns a mill. If a change was ever desired, this might cover a big chunk of the price difference. The 340 front sight looks a little nicer to start with, too.
(I also read something about the rear sight channel being shaped better on the 340 - anyone able to confirm this and/or post photos?)
Titanium cylinder - About three ounces lighter, makes it nicer for pocket carry, a little less pleasant to hang onto under recoil. Cleaning requires greater care not to damage the finish.
Capable of 357 Magnum - Probably wouldn't shoot it much (or ever), but having it and not needing it doesn't hurt anything, while not having it and wanting it sucks.
--------------
Anything I've missed?
All three seem to be available without locks often enough that a patient person can get one.
My overall grasp of it is that, in essence, the 340PD has everything it's supposed to have, and costs roughly what any other typical S&W revolver costs. (I've been there done that on costing out manufactured systems, and it's often the case that simply being smaller doesn't make a system significantly less expensive to manufacture.) The 442/642 is decontented to get the price down to the point that it will still function while being able to compete in the market with Rugers, but it's basically the bare minimum. It functions, but it's difficult and expensive to make changes if desired, has an inherently limited service life of the frame, and greater risk of being sidelined due to mechanical damage. Is that completely out to lunch, or am I at least on paper?