My quick 2 cents.
The P3 is a great machine. Personally I like it's camera better than the Mavics. It's also a difference of $400 for the P3 vs $1100 (with extra battery) for the Mavic. The P3 would be a good entry drone.
What I like about the Mavic.... its portable. If you need something smaller, than the Mavic is great. Perhaps you only walk the drone out to a field and fly. Than you'd not need that portability. The Mavic is much more quite. However, this is not a very big deal. Mavic has forward sensors but to be honest, I don't see this a deal breaker. If you fly the P3 into a tree or a building, it will hit the building. The Mavic will most likely stop. Will you fly right into a tree or building?
The Mavic has it's cons as well. You could buy almost 3 of those P3's for the price of one Mavic. Is this worth what the Mavic will give you? While the images from the camera are of good quality, the field of view is more limited then with the P3. Some people like this and claim is makes for better video. Personally, I like the P3's camera batter.