Bryan X. Chen goes in-depth on the new Apple TV:
The Apple TV may also not be the best streaming device for everyone because of one missing feature: the ability to stream content available in Ultra HD 4K TV, the latest high-definition resolution supported by some of the newest TV sets.
Who do you know that has a 4K TV? And what do they watch?
For early adopters of 4K television sets, the Roku 4, priced at $130, is a better bet. The Roku is versatile, has a nice selection of apps and has a comfortable remote control… Another competing device is Google’s $35 Chromecast, a miniature dongle that pulls streaming content from a smartphone. It’s a good option for people on a tight budget, but I found it unreliable… Amazon’s $100 Fire TV also supports 4K, but I recommend against it because of its cheap-feeling remote control and less polished user interface.
Amazon refuses to sell Apple TV or Google Chromecast on Amazon.com.