The weight of that no-lock, steel-cylinder M&P 340 is 13.3 oz. unloaded, which about splits the difference between the 340PD and the Airweights.
The primary attraction for me of the scandium alloy frame, titanium alloy cylinder 340PD is its very light weight of 11.4 oz. unloaded, as compared to the 442/642, which are 15 oz. unloaded.
I know it doesn't sound like much, but the in-pocket difference seems obvious to me.
Wish I could get a no-lock 11.4 oz. J-frame, but unless S&W decides to make one, I'll just be content with my no-lock 642s and #*@!-lock 340PDs for pocket carry.