The D1 and D2 are designed more for the certified market, as portable devices. That means they don't have any permanent connections to the plane. Thus, the D1/D2 show GPS altitude, ground speed, and ground track. Since the D6 can be permanently installed in a experimental plane, the D6 is able to show standard altitude, airspeed, and magnetic heading. The D6 also can show Angle of Attack if you have our pitot probe with your SkyView.
The D2 adds wifi for display of attitude on a portable device like a tablet, which the D1 and D6 cannot do.
