These sentences are very similar. "Will be" sounds a little more polite or formal to me.

We are watching the movie tonight. Used in the present tense if you are currently in the process of watching the movie although it is frequently used incorrectly to refer to the future action. We will be watching the movie tonight. Used to describe the activity of watching the movie in the future. We are going to watch the movie tonight. Used exactly like “we will be watching the movie tonight” but possibly considered more informal and more commonly used