For submitting video clips, is it the usual cast of characters as far as submitting is concerned or are some sites better sellers than their image counterparts? ie. Does SS and IS dominate the market?

Only IS dominate the market

I don't think this is true. There are Video-only sites which must be do very well because i read several comments in the past that Videoartists earn the same or even more at this sites than IS. I don't have personal experience as i didn't video so far but read about this several times.