If a video/movie/music video is uploaded to Youtube & provided for download (both are the same files), would watching it (streaming) have the same equivalent bandwidth usage as downloading it?
That depends in what quality your are downloading and streaming it, but if they are the same the downloaded data will be equivalent. If of course you will watch till the end.
Since you are downloading the video either way, it would use the same bandwidth. A streaming video is in a format that you can watch AS you download, when combined with buffering. Buffering, as you may know is the downloading of a small portion before the video actually starts. It works when you are using an internet connection that will allow you to download the entire video faster than it would take to watch it. Thus, attempting to watch a streaming video that is HD, on a slow internet connection, would result in the movie pausing frequently, so that the buffer could load another section for you to watch.
But, either way, assuming you are downloading the movie from the same source, it would use the same bandwidth.
Of course, if you choose to stream a video at a low quality, and choose to download the same movie at a higher quality, the end file size of the two would be different. The higher quality would be larger, and would thus require more bandwidth.
The only time where the length of time/bandwidth might vary, would be if the streaming server was only going to ALLOW you to download the data while streaming at a max speed that was based on the quality of the movie... IE allowing a low quality movie to only be downloaded at a lower speed while streamed, to conserve bandwidth on the server end, but to otherwise allow a "downloaded" movie (not streamed, if you were given a choice) to download at the fastest speed available.