Portability is not a significant factor for me.
The Mavic strikes me for all redundant aids and sensors that engineers have implemented into it. I think the Mavic will be safer and easier to "drive" than the Phantom series... but it will do the work for which I buy it ?
Only you can answer that
Another concern I got is the resistance in "difficult scenarios" like taking off and landing on hard terrains... Panthom has a structure that separates sensitive parts of the ground, the Mavic has gimbal and camera almost grazing the ground level...
I see that as an improvement. The Phantom
needs the "legs" because it has a large, clumsy (IMO) gimbal whereas the Mavic has a really neat, small and "tucked" camera. Now, not only is it less exposed, it also helps with GC and stability of the aircraft during all times, above all during take off and landing but also during flight.
Phantom legs are no big help if the AC suffer an impact. I see that much better on the Mavic, as the folding legs are coiled and seem to double as a "dampers" - though it isn't clear by how much (and I wouldn't rely on it anyway), the whole design looks more stable and protected overall than the P3 IMO.
Problems with debris getting in motors and gimbal? I can say for myself, but I take off and land with my DJI quads from grass and mildly-dirt areas, and never had a single issue with dust or anything. I avoid as best I can, and some places are no-no of course, but doesn't really bother me much. If it gets something on it, I'll clean it up, dust off and pack the bird. Sure the Mavic seem more exposed to this for sitting lower, but 1) we can always take off and hand-catch; 2) the gimbal has a protective bubble.
Either way, we'll have to wait and see.