Don Haines said:
To my mind, the two big trade-offs are the AF system and the articulated screen. It would be nice to have both, but.......
The AF point spread on the 5D4 is slightly better, but not enough to make a difference. The AF system has more points, it is more customizable, and the joystick is better than the pad for moving between the points.... this is great if you are after birds in flight, but if not, does it really make a difference if you are not using it?
I find that the only AF points that would matter to me in 5DIV vs 6D2 are the ones slightly higher, right and left of the center zone, basically to AF on eyes, whether it's a human, animal or wildlife. But it's very rare, and usually either taking a step back or zooming out a tiny bit will do the trick just as well.
The joystick or AF points are not useful for birds in flight, in my opinion, nor is iTTR, which is not in the 6DII. However, I think the 5DIV's autofocus is a little faster, as using center point only, I was getting slightly higher keeper rates as compared to 6D2, at the same spot. But because I only had the camera for a few hours, it could just be statistical noise. Still, it felt a little faster using the 100-400LII, but that could be a placebo
In my opinion, for birds in flight, the Center Expanding AF (where it AF in the center, and then expands to the adjacent points if it doesn't find anything) is the one feature I would miss. However, when I was using the 5D4, and the Sony A7R3, which has the same feature, this causes as many problems as it fixes when the bird is against a busy background, or over water, because the camera will sometimes AF on something other than the bird.
For handheld bird portraits, the joystick is nice to move the AF point in the VF to exactly where I want it. But often, if I have the luxury and a monopod or tripod, I will go to live view and magnify anyways, then use MF, to make sure I'm focusing on the exact spot I want (as bird's eye versus bird's belly is a big difference

) In this case, the articulating screen is nice, because you can fold it to avoid the glare of the sun, because often the sun will be behind you, and the live view screen will be very hard to see.