Typically it is a customers choice as to which type of driver camera they prefer. There may be practical considerations which will force the use of one type of driver camera, e.g. airbags in the A-pillar of the vehicle may force the use of the dash-mounted unit. This will vary from vehicle-to-vehicle. From the limited experience we have, the heavier vehicles will tend to go with a dash-mounted camera, whereas medium and light duty vehicles tend to be pillar-mounted (notwithstanding the airbag configuration).
Read more about the various parts here.