I use FS4K in one room and ATV elsewhere.
I ended up buying 2 more ATVs after having used both .
The FS is a great device for the price but other than that, the devices are not comparable.
From a usability perspective, the ATV is very fluid while the FS 4K tends to lag.
There is no ATMOS support on FS for anything other than Prime (which doesn't really count) while the former does ATMOS on every service that supports it.
For local playback, Kodi on FS is just about acceptable but not something that any of the other users in the house will ever use while Infuse on ATV is extremely simple/ joy to use for everyone else as well.
You are right in that The difference on paper between these devices is minimal but the devil is in the details - After all (and given this is primarily an audio forum), a JBL Go BT speaker also reproduces music , doesn't it?