Which would be best - Buddypress, forums or community?

A little background. I have a network of coaches, each of which have their own clients.
Each coach has their own site on the network but everybody comes together in a membership site.
I want to create a place for them to all discuss their stuff.
I need an area where the coaches can all talk with each other privately.
I want an area where all the clients can create threads and talk with each other.

Which of the options of Buddypress, forums or community would you recommend or do you need some more info to make a recommendation?