Sony was probably first out of the chute with an e-book reader but Amazon had such a huge built-in market advantage when it later launched the Kindle that the company almost immediately became the market leader. And it still dominates that market.
The Sony Reader has something strong going for it, however - its books can be read on other e-book readers and books bought, or acquired, in places other than the Sony Reader store can be read on a Sony Reader. Amazon, on the other hand, makes sure that its e-books can only be read on the Kindle device (or with a software installation, on your PC or smart phone) and that, without jumping through all kind of hoops, books from other sources can't be read on the Kindle. For me, that's a huge red flag and deal killer.
Matthew Graven has this to say in The Register:
Some of Sony’s confidence must come from having a certain world power Gin its corner. Sony has worked closely with Google to offer hundreds of thousands of free titles in the Sony’s ebook store. And now Google, another supporter of the ePub format, is getting close to launching its own store, dubbed Google Editions.[...]
In a mocking tone, Haber (president of Sony’s digital reading business division) gibes at publishing companies who have delayed the release of ebook titles. Simon & Schuster, for instance, recently said it may not make digital versions of books available until the hardcover copies have been on shelves for four months. This is similar to the delayed release of paperback versions. "f you don’t allow the content out there, people will find a way to get that content," Haber says. He adds that publishers cause piracy by delaying the release of books in digital formats and that their businesses will prosper if they embrace ebooks.And the plot thickens.
When asked what writers and publishers can do to help promote the growth of an ebook market and their own books, he suggests that they "spend time using the devices out there, experience them, and then think about what you can do differently with digital content that you couldn’t do with physical content." He tells authors they should "think through, perhaps, how you could make your content more interactive.