I have both the Voyage and the Fire HD6. I love both of them, but the Voyage is for long reading sessions much better. The fire is great to have in a bag, but it is heavy for an e-reader and anything but thin, so you would not want to read hours with it.
The voyage is tiny too, but also super thin and light. I read 6 hours with it on a flight last night, with the lighting on at 9, it was heaven. the guy sitting on tbe other side of the aisle was reading on a PW. I could see him staring at my Voyage, but not daring to ask. Just before landing he finally said "wow, that new Voyage looks much better than I have read" Ha

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I also have an Aura HD but I could never managed to read hours on it, the format is jut not pleasant to my hand.