I look at it this way: I started with an older (1971) Model 60 in .38spl., all stainless steel gun. I liked the snubby concept, but it was pretty heavy in the pocket. I looked around for a year or so before finding an Airweight Bodyguard, which I considered ideal. So right now I carry an older 70's Model 38 Airweight. (Alum frame, Stainless barrell and cylinder.) It's pretty light, but only rated for .38spl, not +p, although I do carry them in there, figuring a cylinderful in an emergency won't break the gun.
Looked at the 642, which is cheaper by far, and if you get lucky you can find a +p rated one with no integral lock. Looked again at the 340pd, and it's more expensive, but I have a Colt pony pocketlite that is MA-legal which is a valuable rarity so I'm selling it to fund the 340.
The 340 is a few ounces lighter, and I can still carry ordinary .38 spl, if nothing else, so it's an improvement no matter what.