I've had multiple android set top boxes over the years ranging from MINIX boxes to Nvidia shield TV boxes. Yesterday I got the new Chromecast with Google TV. Maybe it's my years of experience with Android TV versus my lack of time with Google TV, but I find the latter harder to navigate. Personally I feel like Google TV should have an area dedicated to my purchases and my library of movies. None of the recommendations are even similar to what I'm currently watching or my history. I think that UI looks cool but I'm not sure if the user experience is as good as Android TV.
The Library tab is an area dedicated to purchases and your library of movies. You can add anything form your services there by adding it to your watchlist