If you download the video that you would normally stream directly from an online video web site (like Youtube), then there's no difference in quality or download bandwidth used. Videos from many online video sites are limited in their resolution so they are not typically not HD 1080p quality. So if you download a HD 1080p video from the internet (or rip it from a DVD or Blu-ray) and played this HD video, then there's certainly a difference in picture and sound quality. That's what I do. I found online video sites to have poor quality videos for streaming to my nice HD TVs
In the context of downloading or streaming a television program onto a pc, does streaming use more or less mbs? Or is there no difference? Also, does picture quality differ between the two?