Unfortunately, I wouldn't recommend the Xiaomi Mi Box S where 4K is concerned. For 1080p? it's perfect.. 4K? not so much.. Plenty reports on lag and other bugs. For a reliable (purely) Netflix 4K HDR, bla bla experience? i would recommend Amazon (4K Stick or latest Cube..stretch budget) or Roku (Ultra) devices. Even Google's Chromecast Ultra will do a much better job than the Xiaomi Mi Box S. Downside is you need a separate mobile device to act as it's remote. Expected, the Nvidia Shield remains top dog in that department. But sure over-budget.

Sure, Waifus... . Since you've already ordered it. Best to try it out first, who knows they might have pushed an update that makes the Netflix 4K experience smoother. Don't go purchasing another box yet until you can rule out for sure that you want a better experience.

Yup.. Besides their shape & sizes.. It'd be that the Cube has an Ethernet port and serious Alexa integration. The Fire TV Cube is also a full-fledged Alexa speaker. That means that you can speak to it and give it commands without having to hold down a button on the remote (Have to use the remote with the Stick 4K). Now, if you have a smart home with gadgetry you talk to, etc? Then the cube should fit right into that scenario. Else, IMO the stick 4K is good enough. It also supports 4K, HDR10, Dolby Atmos and Dolby Vision. It has access to nearly every streaming service you could want, and the vast Amazon App Store