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That said... instrumental without a heavy beat is more versatile, as it most likely can fit more content, particularly if the content is not being edited to the music.
A heavy beat, though, can often drive the editing style, but might conflict with overlayed voice tracks. Certain instruments used in voice frequency ranges also can conflict with voice.
Vocals are the most difficult to use, as vocals can often conflict with other content. But music with the right vocal might end up being the core of a video's message. Because vocals tend to be specific, though, a musical piece with vocals is the least versatile if you have in mind offering it as "stock music," which is what I assume you're talking about.
[edited by: Robert_Charlton at 6:19 pm (utc) on Mar. 16, 2009]
another you may want to consider, is what type of music resonates with your audience; i've tanked on some videos with music my audience didn't like with nasty feedback.