Choices, especially for 4K Netflix, HBO Go & Kodi:
- NVIDIA Shield
- Apple TV 4K
Need smooth video playback with virtually All Apps, please read this:
Auto frame rate switching & dynamic range matching - 4K (HDR10) capable Hardware (click)
- Apple TV 4K
However as already stated you have to use the MrMC App (Kodi Jarvis) for media player use or a pretty nice App called Infuse - that is really slick.
MrMC cannot install any extra external Addons or use more than the 6 inbuilt Skins due to Apple's tvOS Store App restrictions.
Even tho it is the previous generation of Kodi it runs wells and is snappy due to the surprisingly powerful chipset in the new Apple TV 4K.
YouTube is limited to 1080p on the Apple TV 4K but it can do DolbyVision 4K Netflix and DV 4K iTunes streaming.
Netflix and Amazon Video (& more similar Apps) - DD+ audio on the ATV 4K can support 5.1 Dolby Digital (DD) output to older Dolby Digital only soundbars and TV's. The Shield cannot it needs a HDMI connected DD+ AVR/TV for that sort of 5.1 audio from those particular of Apps.
Need full blown Android Kodi Krypton (& upcoming Kodi Leia) with all the Addons and extra Skin bells and whistles, (non HDR) 4K YouTube, plus other needed Apps as already stated...
- NVIDIA Shield
Shield will give you Atmos and DTS:X Kodi / SPMC passthrough audio if that is important.
Please take particular note of the Mini Wireless remote complaints on the 2017 Shield like this
THIS thread. Such lagging Mini remote complaints seem to be pretty common these days on the Shield.
4K HDR YouTube is not possible on the Shield due to a lack of support for Google's YouTube HDR - VP9 Profile2 video compression standard.
Frame Rate Matching, of virtually all Apps is not possible on the Shield
except when using Kodi, Plex and SPMC - where it works.
Google's Nougat OS on the Shield does not even support colorspace conversion/switching - you have to deep dig into settings and switch manually for correct SDR and then 4K HDR color outputs all the time you view mixed content. (This may possibly get fixed in the next Android Oreo update)
1080p > 4K picture upscaling output quality - noticeable as a users display gets bigger and bigger:
Good on the Apple TV 4K
Below average on the NVIDIA Shield.
Basically choose what is most important:
Full blown Kodi with some video output and DRM Apps streaming audio limitations
or
Restricted Kodi but with a better overall operating system with features and Apps support for proper video outputs and matching frame rates for nice smooth video playback.