The main reason I stick with Amazon is that it has the best integration with my local library, which is where I get 90% of my books. Like most libraries, they are OverDrive based.

I can check out a book from the library on my phone and have it appear wirelessly on my Kindle moments later. Last I checked all non-Kindle devices require you to basically download an .epub file onto your computer and then sideload it with a cable to check out a library book, which is too much hassle for something I do several times a week.