That would depend on what is being viewed/used majority of the time. If it’s gaming the battery li free will drop as it will engage a higher refresh rate. ProMotion usually just balances out 60 Hz in general usage.Adding ProMotion display would actually improve battery life since the display has an adaptive refresh rate.
I found similar battery life between my iPad Pro and iPad mini for my usage.
I ended up using the mini more and pro less due to portability hence I sold off the Pro. It was not comfortable to hold in one hand for a prolonged period of time and once the KB case was attached it was desk bound plus I found I was resting it more due to fatigue.