ONN and Xiaomi are different in that they run a version of Android TV that's closer to stock, and thus is less locked down. As I said, last time I checked the Fire Sticks don't even let you swap their Ad ridden default launcher without workarounds (that they are hard at work in breaking), meanwhile on the Xiaomi/ONN box you just download one from wherever you want and it's done, they also are not doing any "piracy app crackdown" (well, until Google's stupid future Sideloading restrictions come into play)
And yes, ONN is a Walmart Brand.