First, to the OP, go with the Shield for the use case you described. Sounds like you made that decision.
Now I’ll leave this longer review of the various devices that can be used as viewers for Channels. I’m running multiple versions of the major devices at my 6 different TVs, let me explain why I like certain ones for certain situations, and you can figure out what is best for you.
First, don’t bother with the cheaper versions of Apple TV, Fire Stick, or Android boxes. Performance is better on the more expensive versions and the cost difference is insignificant compared to that boost. So Apple TV 4K versus Fire Stick 4K versus Nvidia Shield. I use a universal remote (Harmony), so I ignore button placements but the remote functionalities are key differentiators.
Channels app experience: I find the quality on all three devices is close enough to identical. All are very good, no complaints. Go wired gigabit where possible. It’s just a great app with parity on the platforms, kudos to the developers. The only difference worth mentioning is that the Nvidia shield has additional keyboard functionality in a universal remote, so my wife can use channel up-and-down and guide numbers. This is the reason that the shield sits on the two tv’s that she uses; she missed channel surfing. This was what allowed her to give up Windows Media Center for Channels. As much as I like the Apple TV, the limited remote buttons mean it takes more clicks to do what I want. (If anyone knows a way to use Mac keyboard shortcuts to do things here, like I use Harmony with a Mac device impersonation to give me bluetooth volume control on the ATV, I’d love to know how.)
Now to the differentiators when you get outside the Channels app; this matters and will define the right box for you based on the other things you do besides Channels. The Apple TV 4K is for the TV that my kids use most. The interface is clean, it has the best apps that are most updated, and it is simple to use. The Shield is still at the TVs for my wife, because Channels is the only app she uses. There is a great home screen layout with customized channels where the top line when I turn on the device shows all my favorite apps in a line, and the second horizontal line is my Channels live TV guide with icons for every show in my defined Favorites order, and the third line is my Channels DVR content. From that front page my wife can scroll through her shows and click on exactly what she wants without even launching the Channels app. This is also simple and intuitive for guests, and allows a clean purposeful home screen centered on Channels. I know the ATV has something similar, but you have to choose TV OR DVR, can’t have it both ways. The downside of the Shield is that many Android apps are not updated so this is not ideal if for example you like Hulu or want the new Plex interface. Then there is the Fire Stick 4K. It’s fast and equal to the other two devices even on wifi, but the interface is a cluttered mess of ads, it takes too many clicks to launch the Channels app, occasionally Amazon installs other apps that mess up any Harmony automated launching, but it works well with Alexa for other non-Channels apps (which are updated frequently which is good). The main reason to go with the Fire Stick 4K is cost; it’s cheap and it works very well if you can ignore a cluttered view on the home screen. My kids don’t care but I do. I have this on all of my secondary TVs like at the treadmill.
If you use Bluetooth headphones, Apple TV is easiest, Shield is perfectly fine, but Fire Stick 4K doesn’t allow you to control volume on the headphones, so basically doesn’t work for that use.
For me, I only use the Channels app, so all of those differentiators don’t matter to me, so I like the device by the most comfortable chair.