Another thing to keep in mind: the Fire TV devices have much less storage space than the Apple TV. While you probably won't have a problem running out of space to install apps, the storage becomes an issue with the live TV buffer.
In Channels, the live TV buffer (for pausing/rewinding live content) is stored on the local client device. The Fire TV device will only give approximately 30 minutes of buffer on the high end; if you are using OTA streams, it could be as brief as 5 minutes. By contrast, the Apple TV has much more storage, giving you easily 90 minutes or more of a live buffer.
(Also, the Channels app is developed with the Apple TV in mind. All features are released on the Apple TV first, and the Android clients—including the Fire TV devices—only get the features after.
You are also correct about the Amazon home screen. The crazy amount of ads and pushing of Amazon content is excessive; for me, it makes the Fire TV devices something I try to avoid.)