Technically they take the same amount of data. However I recommend downloading it because you use just as much data watching it online as you would downloading the whole movie. But if you are only watching part of the movie or the movie is short (less than 20 minuets) then watch it online. If the movie is longer than that you will be better off downloading it. It's better to wait 30 minuets for a 2 and a half hour movie and not have to put up with buffering than to have to put up with it buffering every 2 seconds for 2 and a half hours. A good website to downlaod movies off of is http://www.thepiratebay.se For streaming use http://www.netflix.com
It depends. Some sites have high quality version of the same video that they are streaming.
So in those cases the video version for download will be bigger.
Usually the streaming version is smaller than the download version.
Now if you are talking about downloading the video from youtube instead of watching it there, it will be the same thing.
It is the same file...