I have used a number of the tablets mentioned and the Kindle Fire stands out for the price. However as my eyes are a bit older now I find that the display resolution is critical for me, particularly in the sometimes bright light next to a train/ac window. So for me the Kindle HD version would be worthwhile. I have used a Nook for many years a great display in it's day but now completely out performed by the latest Retina and equivalent screens. As you use an IPad you may be disappointed by the cheaper tablet display. My favourite at the moment is the Samsung Galaxy Tab A but a lot more money than the Kindle HD which is very reasonable. To give an example I find the display when surfing on the Nook very tiring but using my sons Samsung Smart Phone with the latest screen is very clear and comfortable.
Keith
Keith