Gosh, it gets really confusing with Android boxes (as anything with Android really):
Android 1: (unofficial) as in the Minix boxes. These are great for people who like to tinker with the box, and is a great XMBC machine (Zattoo is also fine on them), but they can be pretty shit with most other TV apps as they usually have very low quality streams and poor UIs (as they were designed for mobile touch screens).
Android 2: The Amazon Fire TV. All as described above, and some people have been able to sideload GooglePlay apps, which can mean "technically" everything you may want. But like above, these apps were not designed for anything but mobile screens and many refuse to work anyway.
Android 3: The Nexus Player, basically that new Google TV thingie. It seems really good, but not much different to the Amazon Fire TV. Won't have Prime, but may have a few other apps. At the moment, only available in the US, but it may have more opportunity to launch globally than the Amazon Fire TV - as the above box only can ever go to countries which has Prime (and that is only three in a very very big world).
Android 4: Older Google TVs (forget about this)
Android 5: Chromecast. Works great to be honest if you like this sort of half and half approach. Some good services work with it, and from many different countries. But some people have been known to "get lost" as it can really seem confusing at times.
At the moment, a Roku box set to the US has the most American services, so perhaps this is still the best option for those wanting American content, and a separate box for the UK stuff. But I really believe that as long as Amazon don't block this ability to install apps from different regions (using the backdoor method), then this could end up being a Smart DNSer's dream machine.