Chromecast is primarily a great device but it is often used for serving the one major function. Chromecast acts as a receiver, that means if you want to use the Chromecast, then you must possess some sort of device. May it be your Smartphone or computer that possess the software that does support the Chromecast functionality. The phone sends the signals to that of the Chromecast which it passes on to TV.
Whereas Android TV is a bit different type of device. It lets the assessment of the special version of that of the Google play store. It is the best platform for the gamers, though it is capable to perform the same kind of functionality, it is also capable to send its own signal to that of TV. It possesses its own operating system.