The P3P has 4k video whereas the P3A has a max of 2.7k. Stills are the same with either.
The P3P gimbal top (main) board has been known to go bad causing the famous "
no image transmission" and as such it's hard to find working replacements. For this reason, I always tell people to be very careful when buying a used P3P and make sure it has good downlink (FPV) video.
The P3A has a non adjustable downlink (FPV) bitrate of 2 Mb/s versus a P3P adjustable downlink bitrate with a max of 10 Mb/s. This may matter to you if you have a larger screen on your mobile device or if you want to use goggles.
The P3P came new with a higher wattage charger (100w) versus 57w for the P3A.
Despite the post above, there are no sensor differences between them and neither have obstacle avoidance.