Why are so many people assuming that The Sith empire is the evil side and that the Republic is the good side. Bioware have already said that you can be an evil Jedi or a good Sith so from a morality prospective you can join either side. You should pick your faction based on the class you want to play.
OT: I'm rolling either a Bounty Hunter or a Sith Inquisitor first so i'm joining the Empire.