2019 FireTV Cube will run rings around the 2016 era Chipset in the Mi Boxes. You are comparing a AMLogic S922Z vs an old S905X. The CPU & GPU package in the Cube is much more powerful.
Cube is also properly Dolby Audio and DolbyVision licenced, which is important if you want 5.1 audio support from Netflix, Prime Video etc and you have an older DD, only audio receiver.
I have a fire tv 4k stick and a MiBox S. I prefer the MiBox. I prefer less blatant ads and the live channels app for Android TV. You can attach an 500g external hard drive to the box for use as a dvr for it's channels.
Fire cube! Can go handsfree and change channels! Change input by speaking to it. Also 16GB internal storage! Plus remote control through mobile app.
What are you wanting to do/achieve with the Mi Box and Fire Cube?
It’s not possible to give you a proper considered answer without knowing your purpose first.
Do you like seeing Amazon advertise at you? Then go with the Amazon box. I'd rather pay not to have Amazon's stuff pushed in my face...but to each their own.
They have totally different UI's, too so you may like one over the other.