When you stream, you don't use any space aside from temporary memory which your PC overwrites anyway.
Stream is only good if you have fast internet connection. Thus watching it is like a breeze. Also, if you can find a site which will stream the whole movie with good Audio and Video for free, then you hit a gold mine.
Most people prefer to Download because 1.) it's Automatically Free, 2.) Comments provide good Audio and Video reference. and 3.) You can watch the damn thing over and over again until you want to delete it. Besides, movies cost 500 - 800mb only. Not to large. Just download, watch, then delete.