I have used both and found stability issues with both of them when you had certain applications running in the background. So before using them in anger double tap the home button and swipe up on all the other applications.
From a UI perspective AVPlan does win out over OZRunways.
On practicing IFR flying at home on a sim, OZR wins out over AVP.
One feature that drew me straight to AVP for VFR flying was the ability to tap on a Restricted area and getting status information back in an instant.
Both are brilliant products and neither will disappoint you if you invested your hard earned money on them.