I did a lot of research and ended up buying a Kobo for my wife who was looking to switch from paperback. At the time the Clara HD was the newer model in the size and feature family she was looking for. I made the choice based on e-ink quality, wide compatibility of formats (including public library rental), and price.
Keep in mind that Amazon keeps device prices artificially low by pushing ads to your screen. I have had four different Kindle Fires over the years, and hate how intrusive the advertising is on their devices.
After spending a year or so jealous of my wife’s Kobo Clara HD, I bought myself the then newer Kobo Libra 2. Then she was jealous of my Libra and bought herself one, and gave the Clara to our kid who is a bookworm as well.