I have to second the Apex kit for your M&P 40. I put the kit in mine and it actually has a nice reset now! Also the trigger is smoother and lighter.
As far as your M&P 9 goes, the main problem with that gun is the extractor. If you use it for CC get it changed out for the upgrade.
From what I have read, a lot of PD's are having problems with the M&P 9 due to the fact that the factory does not install an extractor designed exclusively for the 9mm round.