! If you want to test your UI in pretty much all environments, a Nexus 10 is all you need.
There are two really useful commands you should know: "adb shell am display-size" and "adb shell am display-density".
These two commands will tell an attached device to emulate a display resolution or density other than the native display properties of the device. You can tell either property to reset by telling it "reset" as the resolution or density argument.
What this means is that with one high resolution, high density device like the Nexus 10, you can simulate the environment of just about any other Android device you'll care about for basic UI testing. The resource system will give you the appropriate resources for the adjusted Configuration and you can check each different device class you want to support in minutes.