I've heard from other people that the Sith Inquisitor class doesn't get nearly as much DS/LS points as the Sith Warrior. Basically the story just doesn't offer as many choices. I've heard multiple people say they were already Dark Side I as Sith Warrior by the time they left Korriban. I'm level 15 and have about 880 Lightside points, 0 Darkside points, and have been using the Diplomacy skill to get about 200 of those points.
Unfortunately th SI class story just doesn't get us a lot of points like SW does...