The real reason is not something very straightforward to explain but AC made that very easy.
You should just use Motor all the time really. There is a bug somewhere that makes the Battery mode not work as intended and because of that is always a very ineffective choice. If they fix the bug maybe I'll explain the good and the bad of each mode.
For a hotlap at RBR, both modes will give same laptime. For any other track or for a race, Motor only.
Ok, I will use the motor then. Hope Kunos will fix the bug though, so it would work as intended.
Thank you for the explanation.