I think there's a big quality-of-life issue here that people are missing regardless of story, and that's server population. Some of the arguments have been valid (unexplored story of the Sith making the character more intriguing, etc.) while some are pure bias ("Sith armor looks badass and Jedis look like wimmins!"), but, just as in WoW, I think a decent sized chunk of the population flows wherever the people are. A more fluid economy, more options to raid/group, more general population just standing around hanging out.
I think a good number of WoW players who play Horde will assume that others will jump to Sith, thereby swinging population numbers that way.
Edit: At least on PvP servers. I could see there being enough of an influx of movie Star Wars fans (who probably trend Republic instead) to shift it possibly the opposite way on PvE servers.