A tablet computer will improve your comfort, not your skills. If your piloting skills can be improved by a tablet, better stay on the ground
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So, choices - tablet means more comfort in cockpit, long term - androids a bit less compared to ipads. You were asking iPad or iPad Mini. Answer is not difficult, which paper size do you use now for your stuff in cockpit? If you use full Letter/DinA4 size, go iPad, if you use half size/A5, go Mini. I do prefer Mini, because it is more convinient in my cockpit. In addition, I do use iPhone6+ for backup and both do their job well.