For Android TV, there's really no difference what version of Android its running under the hood. Visually, there's no difference, tho some people maybe be inclined to think newer = better, but it's all placebo for Android TV.
I've heard nothing but great things with the Shield TV and you really can't go wrong if you can afford it. Hell, this thing is so great, Nvidia kept the same CPU for the 2017 version lol
The mi box with Android TV will still beat out any cheap chinese hack job android boxes.