I started reading digital books primarily on Kindle about 10 years ago. However, a couple years ago I switched to iBooks mostly as a citizen's protest against the justice department going after apple in what seemed like a frivolous case and specially given Amazon's market share - if any company was artificially driving prices down it was Amazon and not Apple.
However, a few months ago, after having read many books on iBooks app I finally switched back to Amazon's kindle app and the corresponding bookstore ecosystem. The primary reason was the terribleness of the iBooks app but there were a couple other reasons too.
So far - I am very happy to be back to the Kindle app and the broader Amazon ecosystem.
iBooks app. My experience has been quite terrible. It would routinely lose my position in a book and put me back on the first page. Specially frustrating because I typically am in the middle of multiple books. On the kindle app I have never lost my place in a book ever. It is an expected behavior in a product 8 years old and yet iBooks made me realize how not all can do it right.
Other than that the experience was generally sluggish and inscrutable. The text highlight feature is very poorly implemented and the drag gesture is often misrecognized as a page flip.
Integration with Audible. The idea that you get an audio book version for some books is great. I think it is time when books are consumed - read or listened to - without a major distinction. Of course Amazon owns Audible and there is something unholy about that concentration of power but also benefits for consumers.
Integration with Goodreads. I like the idea of goodreads as a service that keeps track of all the books I have read or am reading. And with the kindle app that happens automatically. Very cool.
And about that citizen's protest. It seems to me now that the primary argument in favor of Apple was that Amazon is so big so why sue Apple? That is a good sentimental argument but not a good legal one. Apparently Apple did try to control prices by colluding with publishers. No one argues about that. And just because Apple isn't a big a big shot in books isn't a good enough reason to allow them to break the law.
A living ecosystem. Finally, the biggest reason might be that Kindle feels like an app that is maintained and there is a desire in the maker to explore more out of the medium. iBooks seems like another Apple software that is content in providing the most basic service and that too - sub-optimally.
This continues my slow but sure departure from all Apple non-OS software. Robust high-end "prosumer" software is just not something Apple is interested in or good at anymore.