Autopilot will set a direction and use an internal compass to stay in that direction. If you have the ipilot which has the advanced autopilot, it will set a course and use internal gps and stay on that straight line. Normal autopilot will not have this straight line feature... Only the direction. I'm pretty sure minn kota website says this somewhere.
Autopilot only has that 1 feature of staying in that direction.
Ipilot has many features, including the autpilot, such as advanced autopilot, spot-lock(electronic anchor), cruise control and many more.
Also, to let you know, the ipilot on the powderdrive doesn't allow for the operation of a foot pedal. Whereas for a terrova, you can use the remote in conjunction with the foot pedal.
I'm very happy with my terrova. Besides being able to use the footpedal, the foot pedal for the terrova is nicer and the deploy and stow is much better than the powerdrive.