Well i didn't think this day would that i would suggest someone to use an apple device but here we are, thanks to Google's attitude towards AndroidTV.

If price is not a concern, then apple tv is your choice. You will have the apple'ish feel, and it also have better hdr<>sdr conversion and hardware is good too. One good alternative to apple tv is Xbox one S. Similar price, much better hardware, gaming, blu-ray and it has most apps (if you are american).

Choose NVIDIA Shield if you need android exclusive apps.

I do not know Apple TV, but Android TV fulfills its basic functions superbly.

Twitch, netflix, youtube, local player with VLC... All perfect. There is even an app for Mixer that this man did: u/chottomatteee https://play.google.com/store/apps/details?id=com.zackhow.mixerstreamsonly

Shield TV > Apple TV if you like adding apps like Kodi, VLC, RetroArch, file manager, etc.

If you're a basic user who likes a walled garden and don't have to think about it, go with Apple TV.

Go with what UI you like better, or ecosystem you are on. Also see which has all/most of the apps you want.

Depends. If by "this" you mean an Nvidia Shield TV, then yes get this.

If you are thinking any other Android TV device, then get the Apple TV.

The Apple TV 4K is the superior mixed usage SDR and HDR video playback platform. Better for Silky smooth Netflix Playback as well because it's tvOS Firmware has Auto Dynamic range (for correct SDR/HDR color outputs) and Frame rate matching features, and Apps actually use those features:

https://support.apple.com/en-au/HT208288

Otherwise as already suggested go with the platform that has the Apps you actually need.