Apple TV 4K current gen, is nice. Smooth, and fluid.
But, far more limited usage and locked down device, locked to Apple ecosystem of course, as compared to the Shield.
No usb or way to add storage. Only 32gb/64gb models, with no practical way to transfer files to it easily. I could not even find stock of the 64GB model anywhere even through Apple when i went to buy one.
Android TV is far more versatile and customizable and supports far more apps and things, and more, as it support sideloading android apk files.
Apple TV is a simple, easy to use streaming box.
I have found the picture quality when streaming SD content is better. (and perhaps overall in general)
Many apps are just smoother more refined and function better than their Android TV versions. I found like in HBO Max, i watch some old 90's cartoons on there, Android TV it puts gray bars on both sides of the 4:3 picture, not so on Apple TV....and other random quirks among apps that are just better on Apple TV.
The Shield and Googles Android TV (now re-branded as Google TV) i swore buy, UNTIL, they changed the launcher and have crap ton of ads and aggregate content displayed. I went out a bought an Apple TV the day after my Shield switched to that...nice to have a super clean UI.
If you do use a Android TV device for Channels, you can side load the Carbon Themed APK a user here makes, if you prefer a dark with some transparency effect type theme, instead of the Purple default theme.
Either way, both devices work very well for Channels DVR.
The Shield Pro has the additional benefit of being compatible to be used as a Channels DVR Server, though if you do so, you can not add YouTube TV as TVE source.