I have been on the Kindle Paperwhite for almost 5 yrs now and am happy with it. Though I miss the luxury of checking some passage or ref from the pages you have already read. and the advantage is handy, easy on eyes and support dictionary and you can carry loads of books
I use a Kobo Aura. It is very handy and lightweight. Support for external storage and flawless ePub support is its main advantage which made me choose it over Kindle PW.
I had Kindle Fire, I still have it but don't use much now. My wife uses Kindle PW and it's very easy on the eye. It's biggest advantage is that you can completely bring down the back-lighting to 0 in daylight or where there is good ambient light. I have been using iPad Air for last 3+ years. But it's $$$.
So I would suggest Kindle PW + Calibre for format conversion.