My R9 has mostly replaced my M&P 340. As others have said, the 340 is lighter, but I think it prints more when carried in a pants pocket. It's still easily concealed though. I think the 340's double action only is very easy to shoot accurately. 5 rounds of .357 in less than a 6" group offhand at 15 yards is pretty repeatable for me if I keep in practice.
Having said that, my R9 is just as accurate and easy to shoot and you get 7 instead of 5, plus a slimmer unit. I like having both and the R9 is now almost always what I carry, but I'll never get rid of my 340 M&P either.