I’ve had a Nvidia Shield, currently on Apple TV. Both are about equal, but the native app on the Shield is slightly better. On Apple TV I have some problems playing 4k content in the native app. However there’s a Plex client called Infuse that will play anything you throw at it.
I switched from Shield to ATV when one of the system upgrades half-broke Plex for months.
ATV is beautiful and foolproof, also much better at connecting to various Bluetooth accessories, but so limited software-wise (compared to sideloading non-TV optimized apps on Android TV).