I just went from a Hk 45c LEM to a g19. I just updated a thread that I started about going to a 9 from a 45. I had been debating on going from a 45 to 9 for a couple of reasons. I found out that I shoot the g19 better then I shoot the Hk.
I would urge for you to consider a used pre2010 3rd g19 or g17. Back in the day I used to hate on glocks for no apparent reason besides the fact that people who did not even really shoot, owned glocks. As time progressed who knew that glocks would end up working for me. If you are interested in the m&p, shoot it next to a g19 and make your decision based on the fact that you shoot one better then the other and do what the people who have been down this road before recommend and stick with it.
For me I am done with switching guns because I have found the one that works best for me and has the things I want in a defensive handgun.