If you are not into games, then you don't need a Nvidia Shield. I have both Nvidia shield and Xiaomi Mi box S. Both are excellent streaming devices but Nvidia has better GPU for playing games. If you are happy with Your Xiaomi Mi boxes and just use them for streaming, then I would recommend to just stick with them.

I have a Mi Box. Have never tried the Shield. From what I've read, the Shield has far fewer problems when they issue Android updates. Mi Boxes have suffered from a number of issues related to connectivity, audio, lag after updating. Not everyone has the same experience but enough do have these issues with the Mi Box. I've also read reports that Shields are slightly smoother in operation, not markedly but there is a difference according to some.

The Nvidia is the best of pretty much all worlds. Has Amazon Netflix Vudu Hulu and GeForce Now streaming and runs many regular Android apps plus can be a Plex server too.

I have a 2015 Shield and a Mi Box S. The Shield is for sure faster at loading apps, but not by enough to make it work the $170 premium (I paid $30 for my Mi Box S).

• With the Live TV app the Mi Box S shows the correct guide, but the "info" on the main screen is wrong. Clearing the data will fix the problem for a few hours (but at the cost of having to setup your tuner each time). Don't have this issue with the Shield - both running the latest version of the Live TV app.
• Amazon Prime (sideloaded version 4.x on my Mi Box S 8.1) can be finicky and not display the correct aspect ratio on some shows. Playing the same show on the Shield works correctly. (I couldn't get any 5.x version to work on the Mi Box S).
• The Shield can direct play my full Blu-ray rips through Plex where with the Mi Box S I needed a USB / Ethernet adapter to be able to direct stream. (which is fine as it just transcodes the container. Even with a strong 802.11AC connection, it couldn't handle full blu-ray rips, thus the reason for the Ethernet adapter)
• I don't really game with either one so I can't comment there.

TL;DR Just for pure streaming if I told you to tell me which was which (without going into the setup), you probably couldn't tell a difference as they both do that well for the apps I use.

It will depend on what will the box be used for. I use it for Netflix and Youtube. MiBox works perfectly for this two. Have a 4K TV. Audio goes via optical to a old receiver. I may get a updated MiBox when and if it is released. Also have HD Home run on the network. Android 9 broke Fox channel. I have antenna connected directly to the TV. The update did not effect much.

Nvidia l think is more for games and natively works with netflix and other streaming apps, there build in