I've had a P3 4K for 7 months and just got a Mavic.
Given my limited flight time with the Mavic, but an excellent understanding of the features, here's my thoughts:
1. If you want to travel - especially via air - and want to bring your drone, the Mavic is hands-down infinitely more portable than the Phantom. The size difference between the Pelican case I carry the P3 in (with spare props, charger, batteries, etc) is literally 5-6 times the size of the equivalent for the Mavic. I am much more likely to take the Mavic with me where I want to since it is unobtrusive, whereas I'm only taking the Phantom where I need to.
Corollary: The P3 setup looks much more professional - which can make the difference for commercial shoots. With the Mavic and the stock controller and your phone, you look like you are playing with a Nintendo drone toy. This might be mitigated when the remote mount arrives, but there you have it. Sometimes perception is everything.
2. The Phantom, being larger and heavier, *should* be more stable. Anecdotal evidence and limited experience indicates that this may not be the case.
3. Mavic has many more features - obstacle avoidance, bottom camera for more precise RTH landings (and it works!), the tracking, tripod, and cinematic modes are all pretty sweet. I used Tripod mode to approach a tower for closeup shots and between that and the collision avoidance, I was much less worried about an inadvertent crash.
4. Phantom's camera has a much wider field of view - which can be important for photographic work. If you are a photographer, it's essentially the difference between a full-frame and a crop sensor. I did the same job with both earlier today and will be comparing the quality.
5. Mavic's battery charger is safer - no exposed prongs to short out if you forget to cover them as with the Phantom (3).
6. The Mavic's controller covers the home button on my iPhone 7 - making it difficult to unlock. I've ordered a mount to allow use of my iPad - or just to move the iPhone out of the remote.
7. The Mavic's remote charges my iPhone!
8. The Mavic's remote charges with Micro USB - much easier to keep running in the field.
My take - it's going to come down to what and how you fly. I likely will keep the P3 4K for commercial work, and use the Mavic for fun - especially when I travel - and as a backup to the Phantom.
Hope this helps.