I use both apps on the P3P and now I'm testing the Beta Version of Autopilot on the Mavic.
Both apps have different advantages.
Pros of Litchi:
Litchi has a VR possibility,
nice Waypoints and the possibility to plan them on the Desktop + batch functions ...
Litchi is closer to the GO app an therefor easier to use.
To me, Litchi is better for free flying (google maps ...)
I prefer the Panomode of Litchi a little
Litchi missions can continue with signal loss without limitation. Autopilot has limits there.
Litchi works on Android an IOS
Pros of Autopilot:
The main reason I use it, ist the Follow funktion, witch is much much better and smoother.
The possibilities of different focus strategies for semi automatic flights are way greater then with Litchi.
Autopilot allows a grid mission. (there are other apps fo this as well)
I personally love Autopilot and I think its strange, that there is so few discussion about it in the Web. And its not like its so super complicatet, that you have to be almost pro to use it. And the Youtube tutorials are awsom.