A:Answeripad air 4 (5 or 8 aren't out) has a larger screen, although the 2 are the same size. the air 4 is much more expensive, so i would recommend going with the ipad 8th gen. i'm going to assume you mean ipad air 4 and ipad gen 8 :)
A:AnswerApple products don't use SD card or any type of external memory. You can buy storage from the icloud. I think you can do some type of monthly payment for 5GB or 10GB depending how much space do you need.