For viewing, personally I use YouTube - its easier to browse from one video to another, IMO. For uploading, both, plus any other video sharing site I can find. Most of the pages I came across in researching for the OP indicated that in analyzing the statistics for most (non-viral) videos, optimizers and producers found that the biggest source of views was from "related videos" links, rather than from search, direct traffic, or any other source.

Building on that, the more services you distribute the video on, the larger the audience you can attract to the video through "releated" viewing. In addition, as search engines index videos from ever widening lists of video sharing networks, your video gets more opportunities to be displayed for more searches.

These factors may also increase the importance of posting the video to sharing sites, as opposed to submitting links to your video page to video search engines. It may also be a factor to keep in mind when creating your tags for the video, so that it can be properly associated with other popular videos of similar subject.

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The best way to learn anything, is to question everything.