I really think that Jedi and Sith will be the number one and two spot for most played classes but I don't think that the gap will be that great. I was out with my buddies a few weeks ago and out of the six of us only one had plans to make Jedi/Sith their main. The rest of us are far more interested in the other classes and what has not been announced yet.

In the end though people gravitate towards what is fun to play which at this point we can not predict.