Help with M9

In last a few years I've used Nikon D700. Great IQ. But now I am considering switching to M9 because on my back I have D700, 24-70/2.8, 80-200/2.8. It's so heavy.
I've asked friend's of mine who work in a camera shop what they thing about it.
I photograph mostly landscape, portrait, my family, children.

They told me that Leica M9 is made to be use primarily in low aperture. They said to me there are problems to take a sharp picture of children. I don't want to shot sports or something like that. I know that M9 requires different attitude. But what I can't do with M9 compared to D700? I am really confused.
Thank for comment. Radim