For Mavic:
Radio link range is greater, battery last a couple of minutes longer.
It's significantly quieter and generally less intrusive being smaller form and darker coluor.
Superfast setting up and packing away.
It looks less like a plastic toy...
Additional batteries are significantly cheaper.

The reduced size, weight and portability cannot be emphasised enough.
With a couple of spare batteries and controller the Phantom backpack is quite heavy and decidedly cumbersome.
A mavic will literally fit in a large pocket and the controller will go in a sunglasses case.
It's a bit like comparing a desktop PC with a laptop.

Phantom:
More tried and trusted established platform, lots of spare parts and add ons at reasonable price.
Does not require you to ensure focus is more or less on infinity before you start recording.
Field of view is wider, default settings are somewhat sharper - not always good as can encourage annoying artefacts e.g. on tiled roofs.
Phantom 4 has many of Mavic advanced features such as obstacle avoidance.
You can easily use a larger tablet on the controller.
The camera and gimbal is held further from the ground which is useful taking off in the field.
Easier to hand catch safely by the leg when no hard surface to land on.

Mavic is probably easier as a first drone even though in some ways it is more complex.
Considering Mavic is similar price to Phantom 4 if not cheaper it's almost a no brainer.
However bear in mind whatever looks like a big jump forward becomes old in a year or two at most with these.

Probably by 2020 you'll pull a ball shaped thing from your pocket, throw it in the air and it will instantaneously unfurl - transmitting live 4K feed to your computer at home!