iiiNelson It’s actually very simple if we keep out the tech behind each function.
Cine EI is simply a LUT and the analogue science (analogue voltage etc ISO) remains the same when you change the EI settings.
Thus when changing EI iso you only push u brightness of the other way around, but the dynamic range of your camera remains the same. Both clipping point and shadows are the same. A good practive is to set zebra to 99% then dial in your exposure until you dont clip the areas you want to preserve and then set the EI value for how bright you want to see your image. Then check if your shadows are clean enough and if they arent’ kill more of hyour highlights. The zebra is set to individual RGB so, it already starts to zebra when for instance GB are still good, this is a good thing becaus eif you start loosing one color channel you get color skew.
On the other hand ISO mode, is complicated and not worth investing your time, if you familiar hyourself with cine EI.
One note about CINE EI and dual native ISO. One thing that you have to keep in mind, and you can easily see this effect yourself.. is that when you go from 1200 to 1600 iso in cine ei, is that all of a sudden your clipping point is taken down, so a lot more clipping, this is because when you go to 1600 you shift to the 2nd native iso base. So in daylight stay away from 1600 and at night stay away from 1200 or below. Hope this works for you.