I have one of those Android "TV Boxes" from Ali, and can't stand Prime and Netflix in such a shitty resolution anymore (especially because I pay for most expensive Netflix package). My TV is 1080p, not 4k, so don't know which one from these two is better for the long haul?
I live in Serbia, we have official Xiaomi store, but would probably buy either one of those from local online re-sellers, and price for both is similar. Apps I need are Netflix, Amazon Prime, Twitch, Chrome, and HBO Go, local version of HBO streaming app. Nvidia Shield is not an option, it's too expensive.
I'm not a fan of the Fire Stick. It always feels like it's trying to sell you Amazon content, rather than enable you to watch content across all your services.