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Light is a zealot on the side of righteous justice. Lelouch is a rebel with a strong attitude and a deep seeded and well justified hatred of the Britannic Empire. Nothing alike. They both plan, yes, but Lelouch does it so much better than Light.

They both attempt to create a better world but through completely different methods and for completely different reasons. Lelouch goes for an all-out war against the Brittanian Empire and, for some time, the world and he does it for a selfish reason, allbeit, again, justified. Light takes on a more sniper-like ideal. He doesn't want to be seen and is running a very high risk all the time but there are things he could have done better, much better. He does everything he does because of a warped sense of justice and his weapon is not for personal gain, the Death Gods have nothing to gain from killing people.

That's like a square though. (It's a variation of a rectangle and a parallelogram.) Some Death Gods kill those they don't like but they have to have a condition, they have to not like someone. The humans that use them first have to gain them first therefore already have a condition before using it for themselves. Otherwise it's not something that happens.

The immortals can't use the geass themselves. Also, the only person with geass that I see not using it for a selfish reason was Rolo. Lelouch does it for a selfish and justified reason all throughout the series. (At least, that's what we think until it's shown that he did it for Nunnaly at the end of R2.) And as for who's more powerful? It depends on who knows about who first.