I know this might not be a good forum to post. I'm a bit dji fanboy myself... but I must admit I am pretty amazed by what the new Xiaomi 4k can do.
I have been asked many times to compare the two and I finally got to it now.
Here's my recent youtube video:
Camera
I know that I'm pretty undecided when it comes to the camera of each... I like the colors on the Xiaomi better and the other specs... but I feel that there is a better dinamic range on the Phantom 3 (am I wrong?).
I also like that Xiaomi records 2.7k in 60fps which is pretty amazing... and 100fps for full hd.
The fact that it has no cables lying around near the camera is also a plus for me.
Transmitter
Xiaomi has the smaller, simpler transmitter, but I like how the sticks feel on the DJI Phantom 3 better.
And what can I say overall.. They have very similar performance... but if you get them both new... the Phantom 3 is considerably more expensive