I don't think this is a valid question. If it is exactly the same video file then there isn't any difference in the volume of data downloaded or streamed. Streaming video on YouTube is a misnomer because it is really Progressive Download[1], i.e., video is downloaded in parts as the user consumes the video. If the user pauses playback then the download also pauses. However in a traditional download, the user is unable to playback until after the file is downloaded.
So in case of streaming, the user's web browser downloads media slowly as it is consumed while in the case of traditional downloading the media is downloaded as quickly as possible so that the user can start to consume it as soon as it is downloaded.
Progressive Download saves bandwidth costs for the streaming company (like YouTube, Hulu) because if the user skips to another file then only a part of the initial file is downloaded. Furthermore, it may save money for the user if they pay based on their usage.
Another big advantage of progressive download and streaming over download and watch is that you can watch while the media is only partly downloaded.
