I've never had a Kindle: I started with Sony, and went over to Kobo with the Kobo Glo. I read a minimum of 4 hours a day, have problem eyes, and I buy into any significant improvement - always with Kobo.
For myself I wouldn't buy a Kindle, but I do recommend it around me for non tech-savvy people (mainly the parents of the friends/family who ask me), and they always tell me they are pleased with the choice.
But I dislike Kindles' very poor typography (see
http://www.cgpgrey.com/blog/voyage-to-nowhere), the poor font choice (number of fonts, with no possible adding, number of font size choice, weight adjusting), and Amazon's proprietary formats.
My main sources for books are Amazon and Kobo for a similar %, ebooks.com, ebook.de and feedbook for half of this %. I choose according to pricing (I'm French and buy from amazon.com, so there is no price compensation from Kobo - books price is fixed in France), but I've had to cancel books from Amazon and buy the Kobo version, because the Amazon version was unreadable. This was mostly non-fiction(no links to notes for instance), but I have a 220 pages fiction exemple in 4 parts, each with 4 and 5 chapter and a glossary, which in the Amazon version had no separation whatsoever. Also a book where all the poems were images, unreadable. So I only buy from Amazon when the price difference is significant, because the refunding costs a little (because of change charges both ways), and I prefer buying epubs for html quality sensitive books like poetry, where Amazon converting the publisher book and me reconverting it in an epub can make strange beasts.
In any case, I would NEVER buy books from any reader: I want to own the books I buy, so I import them into Calibre to get rid of the DRM's, correct them in Sigil and polish them in Calibre. And then I send them as kepubs to my Kobos to have the best look for me: header, footer, margins, fonts, plus the advancement bars.