As a person who has used both extensively:
The Shield is better at sideloading. So if you use apps like smart tube next, or other apps like cinema HD, Bee TV to tap into torrent streaming, Kodi for its addons, or real debrid streaming, then Shield is the way to go. It’s got better support for those types of apps.
I’m a heavy Plex user but use the Apple TV 4K (latest gen). Both are great for Plex but if you have a home theatre setup, the Shield is better because of its TrueHD capability.
The Apple TV has a faster UI. Shield is fine but it can definitely lag in places and my Shield is prone to crashing from time to time. Mic on the Siri Remote is the best I’ve seen on a remote. The Google Assistant has never worked consistently for me.
If you’re in the Apple ecosystem, the ATV4k has nice support for Apple AirPods which integrate very well.
I’ve not been a beat-em-up player for a while so when I saw these new “fight sticks” I was fascinated because they’re supposedly the new hotness (well for a few years now).
My mate and I really got into SFIV back in 2009 and we got decent arcade sticks and bought Samwa parts and did them up, then played the crap out of SFIV and were still shit at the game. Good times.
But yes the Deck has great controller support. It has recognised nearly everything I’ve connected to it. Will definitely check out Fightcade.