Light would never have been capable of 'taking over the world' even when he didn't share a reality with Lelouche, his goal was to usher in a new era for the world in which Kira was the god. There's a difference between people fearing you as a faceless god of death and directly ruling over the world. The death note isn't really an effective tool for straight up taking over the world, as you'd be ruling through fear-tactics with no loyalty and the only thing protecting you being that people don't know who you are (And Light already proved in the show that he couldn't keep his identity secret forever).
So yeah, Lelouche definitely. Though it would also be a likely possibility that it would end in a tie, with Light killing Lelouche but ultimately failing to take over the world and being killed by the people.