The Mavic 3 camera is newer tech, has a larger sensor and larger pixels. It will be able to eek out a little more detail in general and will work quite a bit better in low light situations than the Phantom 4 Pro for photography. You will get a variable aperture with either drone but you give up the mechanical shutter by going from the Phantom to the Mavic.
For video the Mavic 3 will have quite a few improvements in store for you. 5.1K resolution, higher bitrate, higher available frame rates and refined automated flight modes. The same benefits and deficits listed above for photos also apply to the Mavic 3's video ability.
In general I feel that they are both great aircraft and there are still benefits to having a Phantom 4 Pro. If you are comfortable with the Phantom and it gets good results for you then the only real reason to upgrade is because you want to. The Mavic 3 still has some bugs to work out in software so even if you decide to purchase one, it might be good to wait a little longer.
