If you want the quick answer to what UAV you should get... here it is: Get a Phantom (the older version DJI is still selling will work as long as you stay away from the P3 Standard!). Yuunec and 3DR don't have a get community of 3rd party apps to use and/or they don't geotag photos out of the box. If you're on a budget, get the Phantom 3 Advanced. If you want to have more flexibility and greater detail, get an Inspire. If you want slightly more power / more features, by all means, the Phantom 4 is a great pick, but for mapping it isn't doing a whole lot more than the Phantom 3 Professional.
The things you'd ideally want:
- 3rd party apps to automate your flights.
- There are multiple solutions for DJI, and only 1-2 solutions for 3DR (if you want to count using Mission Planner apart from using the Tower app).
- Photos are automatically geotag.
- You can geotag photos after the fact, but it can be frustrating and time intensive. Currently 3DR doesn't support this out of the box with the GoPro you also need.
- Flat lens.
- Curved/distroted pictures decrease quality and introduce errors. DJI products no longer have "fisheye". You you must use a GoPro, either upgrade the lens, or be prepared for some editing of the photos in post.