I had a Firestick 4K then swapped it out for an AppleTV 4K. The Apple TV UX is really clean, fast, and lovely. The Fire TV stick had adverts and stuff in it and the UI, while simple, felt cluttered and a bit confusing in comparison. Fire TV just felt like an amazon storefront and it was difficult understanding what I did or didn’t actually have access to. On Apple TV, your content is featured first and foremost, and it recommends shows to you based on what you’re subscribed to rather than what it thinks you’ll spend money on.
The downside to Apple TV is that apps that haven’t fully integrated (namely Netflix) get kind of swept aside. There is no side loading and no browser, so you can’t get unofficial apps running on the system like on Fire TV. (I used to have a side loaded app that just let me stream movies and shows from some sketchy, foreign website) There’s also no VPN for Apple TV currently, but that’s coming with the next major software upgrade.