Currently in my bedroom TV I have a 2nd gen Chromecast. The TV is only 1080p, no HDR, only using the built-in speakers, it’s LG so the CEC has never worked. I wanted to buy something with a remote, but I don’t like the Roku UI that much, and the Apple TV is too expensive for my usage.
All the apps I use are available for Android TV. The only ‘special’ use I would make is buying a TV Tuner (MyGica PT275) just so I could watch everything under the same UI (my cable provider sends unencrypted ISDB-T television). With the CCwGTV I’d need a dongle, with the Mi Box only a USB adapter.
I'd really go for the Chromecast. It has a newer UI, Android TV 10, a much better SoC with an improved GPU (and CPU).
It's also smoother, it has a better remote in my opinion. They added kids profiles recently, so who knows what coming next, I think Google will constantly improve it.
The Mi Box S hardware is almost the same as the Mi Bix, which is from 2016! That's old stuff.