A lengthy post but glad you asked...
The first 2 generations of OLED had some major issues - motion, horrible near black handling, brightness and vignetting. That is why I waited for improvements in OLED technology (and hoping Samsung would join the party). 2016 OLEDs resolved most of the issues to a great extent.
E6 has very little black crush (not sure if that is the correct term) that you can minimise after adjustments. Motion resolution is just fine. I am hardly be affected by motion or judder issues. I watch a lot of movies; both blu-ray and streamed. They are really good, mostly jaw dropping. Let me know if you want to see some pictures from streamed content. I come from Samsung 64F8500 Plasma and do not miss it a bit.
B6 has a different SoC compared to a better one on E6 and higher. I did demos of B6 and E6 side by side. Sales guys said no difference apart from 3D and fancy sound bar on E6 but I could see clear difference. So I went with E6 which, to my eyes, was superior enough for the increased price. Later I read about the SoC difference and happy with decision to buy E6.
2017 are much better in near black handling and to lesser extent on brightness. I demoed C7 - very short one. Note that C7 panel is very reflective and could be distracting in well lit rooms. E7 will be better in that regard though I have not seen in person.
There will be macro blocking and noise on dark compressed sources. But if you want a cleaner picture then Sony is better. I did not demo Sony OLED but am aware of the smooth gradation feature that works wonders with compressed sources. But I think Sony OLED will cost a fortune
I would choose between E6 and E7. C7 requires a light controlled room. Motion remains same on 2016 and 2017 models except that B6 is a bit worse.