The Apple TV will scale-down the movie to fit your TV. But if your TV is 720p you're not gaining anything by purchasing the 1080p version, unless it's future-proofing if you buy a bigger TV in future.
The Apple TV will scale-down the movie to fit your TV. But if your TV is 720p you're not gaining anything by purchasing the 1080p version, unless it's future-proofing if you buy a bigger TV in future.