You can game stream on Mi Box perfectly good. I have sideload GFN Android TV app and streams very good with XBOX Elite controller using bluetooth.

Nvidia Shield is very powerful if you want to run dedicated PLEX server, game emulation, play exotic video codecs, generally for extra (geeky) mile. It's overpriced and repacked the same old processor as new versions three times since 2015. Every time they remove some feature and value goes down. Also it doesn't support HDR YouTube.

The Android TV is so optimized that runs very good on low and fast hardware. The difference between them on opening apps is small for example when opening an app like YouTube Shield is 5sec faster.

Try first the $60 Mi Box and then go to $300 Shield Pro 2019. I have both. Happy streaming.

One of the big difference between the 2 you'll notice is the remote. The mi box doesn't control the actual tv volume just on the mi box. You'll also not be able to turn the tv on/off. With the nvidia shield you'll be able to do both. The shield just also seems a lot smoother in operation than the mi box. I'm keeping the mi box and putting that on my 2nd tv that i use on occasion and keeping the shield on the one i use the most. Both are great boxes. If you don't mind having a 2nd remote to control volume and turn the tv on/off than the mi box is a good choice. If you want more then go with the shield.

i've found this guy does excellent reviews and he is a shield user so that is often what he compares new devices to. Here is his 15 minute review of the mi box s https://www.youtube.com/watch?v=qTQtVgo6aWI

I use a 2017 sheild tv. And i use it for Plex, netflix, twitch, youtube, youtube tv for live tv, prime, and a few other specific tv network apps. Mine is an older version than is out now but what i was paying for was speed of the interface. There is zero lag when i'm scrolling in menus. I think that is because it has a stronger processor and more ram than other devices. I bought it because it has an onboard plex server and i can stream all my videos (large 1080p movie library) from my networked hard drive without having to run a server from another computer. I'm in a small apartment and don't want to have another computer running 24/7 plus now my computer is my macbook and i don't want to always have to use it as a dedicated plex server. I'd rather use it while on the couch watching tv. Another key in my shield tv buying decision is also it had wired ethernet. I don't use the wifi on the device. My internet is only about 80mbps, but i have no buffering on movies or netflix. My old firetv occasionally buffered over wifi. My shield also has two usb ports. I first used one for a dongle for a 3rd party remote (now use a firetv with volume remote which i prefer. Since it's bluetooth it's compatible). But i also have an external ssd attached that allows me to put my plex server on that and install many apps and games on it. Granted i have no games on it. I game on a ps4 but i did have a two month trial of stadia that ended last month and i was able to stream destiny (on shieldtv) without many issues. i know you won't be doing this but that was sort of a stream of conscious comment lol. i don't game on my device anymore. Another thing to mention is my device is 3 years old and i still get nvidia updates and honestly i can't think of a single device that beats my three year old shield that's not a new shield pro. Which is pricier than mine when i got mine. i got mine on sale i think for $130 i think in 2017.

But my advice is definitely know how you want to use it, the ports you'll need, will the ram do what you need and then you'll have a good idea if the price is worth it. Cause it might not be. Like my parents have a roku. All they watch is netflix. They could use pretty much any device. A plex would be overkill. For me i had a firestick and it couldn't do what i wanted well enough. Again for me the shield was worth it cause i needed, ethernet, extra usb ports, more power, on board plex server. it might not be for everyone but i'm more than happy.