It all comes down to what diversity you want for books, and what weight you want in your hand when using your device. Any of the mini-HD devices are heavier than a Kindle itself, being pretty much just for books. If you want to hold a mini device, like iPad Mini, Kindle Fire, Nook, and so forth.
The dedicated readers are light. Paperwhite is brilliant. I have one, and that lighting system is far better than reading on a backlit display, such as phone, iPad, Nook, tablets in general. Backlighting often will hurt your eyes after a period of time, where the Paperwhite has a top down lighting system in the frame, pushing the light into the screen not out of it, at your eyes.
It all comes down to budget and who you want to buy from. A Nook does a lot more open formats, along with Banes and Nobles format, but won't do Kindle. Kindle will do lots of formats, but not Barnes and Nobles Nook format and a few others. The Nook is more diverse for buying, however; that doesn't mean you're getting cheaper books... just more diversity from where you can get them. Amazon you're pretty much limited to Amazon for books.
I search for all my books first, as some are double the price to buy electronically, others are a reduction of the paperback price to buy electronically. You really have to look on a book by book basis.