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I really do not think it matters except for screen size. I have always found that in the car or if in a room I have it plugged in while I am watching a movie. I can not speak for the iPad Pro but can for the iPad 4 & iPad Mini, both of which I have found good for viewing a movie with. I have shared a movie on the mini on an airplane which I thought was the perfect size, whereas an iPad 4 was perfect for sharing a movie in a vehicle.
What I would suggest is think about where you are going to use it the most and what screen size will best accommodate you. If you have an Apple Store close by, maybe stop in and have a look at all three sizes to compare, might help you with your decision. Hope this info helps you.