AP Is best to hit the planes in the air, being AP flies MUCH straighter - but learning to hit planes are a different story, lots of practice.
HE However can be used, but you have to "lob" it at the plane, its much better to take a split loadout if your attacking an airfield for this cause.
I always Use HE to take out planes on runways or ord bunkers, and still have AP for Tanks outa the VH or even aircraft taking off.
Just remember if you spit a HE close to a plane taking off it will blow it up. AP requires direct hit, but if the planes on the ground it will most likely take the tail, wing off and plane still alive.