Just like many folks are locked into Apple's walled garden, I reckon many people, especially Kindle users will actively become a part of the Amazon prison.
And the term "Amazon prison" is not my own but hose of Shelley Palmer, blogger. And he makes a great case for it. And while the term "walled garden" being applied to Apple's ecosystem sounds better, we have to wonder if they are both the same.
Amazon's contents are no necessarily exclusive to Kindle hardware. However, Apple's contents, music and video, can only be used with iOS and OS X hardware, basically Apple gears. What's interesting is that you can go arguments for and against going with Apple or Amazon.
Palmer did a good job arguing for Amazon. It's a good read. At the same time, he's talking with respects to content, not hardware. And when you bring in the hardware equation, Apple and other hardware makers also have a great argument to make.
So, what matters to you more? The content or the hardware?