Both. It depends on the game type. Originally, when I first got Jedi Knight 1, I played light side. I later came to think the dark side was more powerful, and I used that for a while. Many people died at my hands. But now I find the light side to be most useful.
In Capture the Flag games, I usually use the light side. Heal, Absorb, and sometimes Mind Trick are great force powers for both defensive and offensive needs. Sometimes I use the dark side, though - it depends on the map. Grip is great for throwing attackers off ledges and the like.