I haven't completed all of the storylines, but I've gotten through to chapter 2 on everyone, so here's my two cents based on that
The Imperial Agent (Sniper/Operative) has a fairly interesting storyline, I think. They get more of a perspective of what it's like in the Empire working with/against the Sith. It also has a very "James Bond" feel. The Sniper gameplay involves the cover mechanic, which I personally don't like, but basically you can click a button to take cover. If you take cover behind the environment, most ranged attacks miss you as long as you stay behind cover. Other than that, you're a ranged dps class.
The Jedi Sage storyline felt fairly bland to me. "Go here, heal this master. Ok go over here and heal THIS master. Then go heal this one, then this one, then this one." Plus I don't care for Qyzen, and he's your first companion and also a tank, so I usually end up running around with him.
The Sorcerer storyline was much more interesting to me. Force Ghosts and extra Dark Side power over said Ghosts. I enjoyed it. Also, Khem is much more fun to me.
As far as gameplay, the Sage/Sorcerer are the same class, so it's the same with different names on the abilities. The Telekinesis/Lightning path gives you pretty good AOE damage and plenty of proc abilities. You also have a healing path. The other DPS path is the "shared spec" that is the same for Sage/Shadow and Sorcerer/Assassin. I don't know much about it, I prefer to stay away from the shared talent trees, but from what I hear it's pretty good in PvP.
Cheers