Ted is pretty much dead on.  But, as of this fall the Trimble Field Manager can work with 3rd party controllers.  And the Field Manager can control liquid applications directly with the EZ-Boom module.  This module also gives swath control.

I have the Field Manager with autopilot in a MT765.  We did application control thru a Mid-Tech 6100 this fall (The Field Manager send rates to the 6100).  As Ted said, the hardware is the same on the InSight and the Field Manager.  But, the software is proprietary to each company.  The Field Manager has more autosteer features (some will probably dispute that) than the InSight, but  the InSight can do yield monitoring and has more machine control capabilities.  The InSight is could be considered more versatile.

If you don't need to do yield monitoring and autosteer is the main concern, I would look at the Field Manager.  If you want to put the unit on a combine, the InSight is the way to go.  You can do autosteer and yield monitoring at the same time from the same box.  Dealer support would be a big consideration also.