Spec wise, they're nearly the same system. A minor SoC tweak, but basically the same processor/GPU/VPU.

The main thing you should look at, imo, is the app lists and the UI. Preferences will be different per person. Hardware wise they both should run apps fine, only "big things" would be if you need USB (for flash drive or hardwire ethernet) or if you need IR for universal remotes.

Depends if you're an Amazon prime member and would like to watch from their catalogue without a work around.

I have 3 mi boxes with one that has the Netflix issue. Also considering new fire TV

I would recommend the Mi Box it runs really well and it's my first Android TV box. For me it does 4K 60hz without HDR due to my TV only having one HDR port and I need a switch.

The Fire TV is good if you want to watch amazon prime video without workaronds.

I have both i like the Amazon tv box better and it has most of the same apps as the mi box. Also the Amazon tv as a gigibit Ethernet port and the Amazon box will switch to native 4k or 1080 automatically and the mi box is kinda set it and forget it.

The diferences are in the video codecs and audio codecs supported. I dont know if they are OS level issues or they can be solved by an app or software decoding.

According to Amazon, the Fire TV only supports VP9, HEVC/H.265 and H.264/AVC

The other codecs are not supported. I dont know if they can be played by an app like Kodi/VLC.

It goes the same with the DTS audio codecs. They are not so used when it come to streaming services(most of them use DD+ or Atmos), but if you stream your own library using a NAS or Plex, this may be an issue.

Since i am not a power user, I would chose the Fire TV 4K Stick if they were 100% supported in my country, but here the Mi Box is a better 1080p choice.

Depends what is important to you. I suspect mibox will get assistant with in 6 months.