I think a lot depends on whether you want to read text only, or whether you also want to be able to have the text read out loud to you. From the research I have done online, the Kindle paperwhite seems to provide the easiest on the eyes way to read text indoors and outdoors. However, I just downloaded the Kindle app on my iPad air and find that I can use Apple's VoiceOver option to have the iPad read a Kindle book to me. VoiceOver is one of the built in accessibility features of the iPad. The voice is synthesized so it is not the same as, say, listening to an audiobook from Audible, but also not bad.