We were recently asked to put the Mavic and Phantom 4 in a head to head trial over in Maldon in Essex.
We found the results very interesting. The Mavic being much smaller was much easier to carry to location ( we had to walk around half a mile ). The Mavic was just thrown over the shoulder via a shoulder strap and carry case no bigger than a typical camera case. The Phantom 4 however comes in a much bigger and heavier polystyrene carry case. The phantom 4 in its box does however look a little more substantial and commercial looking in our opinion.
Performance wise, there was very little in it. Both were very quick to respond and very smooth to fly. Both reached similar heights at full speed pretty much simultaneously, the Mavic being slightly quicker ( not that speed should really be a deciding factor in our opinion ).
Both take outstanding footage.