This is kinda hard to decide, just because the way you describe the situation is a bit complicated. Mixing the two different types of strategy required between Geass and Death Note doesn't go together too well and it's really hard to say how things would turn out with something like that. Light and Lelouch specialize in two different areas. Lelouch has skill when it comes to charisma and masterminding people under him to bring about revolutionary change through military strength and ideological motivation. Light is a lot more isolated, having only a few people with him for a more psychological mystery-solving brains battle type. Mixing both of those into a simple "who will win" type of battle isn't quite so simple and honestly it could go either way depending on how things play out. I cannot imagine Light managing to identify and kill Lelouch so easily, and I forget if Misa's Shinigami Eyes work past the Zero mask, but at the same time if he had Britannian aid then it would make it a lot harder for Lelouch. It's too complicated for me to say a definite "Light wins" or "Lelouch wins."
However.....
Light Yagami WAS smarter then Lelouch. Light Yagami, using only the Death Note, with absolutely no access to resources or an army, managed to make the whole world bow to him. HOWEVER, in the last episodes after managing to defeat L, Light Yagami lost his touch. He acted carelessly and let a child nearly half his age beat him. Light Yagami found a rival in L and after L died, he found none else to be worthy. He got cocky and careless, got arrested, went insane, got shot, then bled to death in a ditch somewhere only to be overlooked by his former rival in spirit. Lelouch however, achieved his goals for a better world because he never let the power of Geass influence him. He'd already been making plans to change the world even without the Geass, as impossible as it may have been. However, Lelouch used his charisma to gain an army and if it wasn't for that army, his plan would've failed. Light Yagami manipulated people around him and used only Misa for his new world. Needless to say had he not gotten careless with Near, Light Yagami or Kira would've been worshipped as the new God.
I'd have to disagree with the idea that Light was smarter than Lelouch. He had some good plans, sure, and no doubt his maneuver to erase his memories and get them back in a complicated situation was a very well thought out plan. But the problem is that this is his only really amazing show of intelligence. The rest is filled with him being incredibly arrogant. You say that his pride ruined him in the end, but the reality is that Light was bogged down by it since the very beginning. The instance of writing Lind L. Tailor's name in the Death Note, despite it being such blatant bait that a middle schooler could've realized it, was a shameful case of his god complex and the fact that he thinks he's too good for the rest of the world. I can't say that he's a stupid individual, but his pride makes him do really, really stupid things. The fact of the matter is that the only reason Light was forced into a situation with L around in the first place was BECAUSE he was arrogant and wanted a challenge, wishing to crush anyone who opposed him. The most practical way would've been to avoid doing anything to catch such attention, obviously by not falling for stupid bait, but even before that, by not making it so obvious that someone was behind it all. Obviously that wouldn't happen because of the fact that his entire motivation for doing this was the reason why he was so extreme in it, but the most intelligent way would have been to opt NOT to do that and to be much more subverted in his actions.
Lelouch on the other hand never made such terrible blunders. Sure, he was arrogant at times, and his plans didn't always go as perfectly as he wanted. But his pride was never as blatantly idiotic as Light's and if anything, a lot of the circumstances that caused him trouble were things that were unforeseen issues out of his control, like with the Euphie Geass deal. Not to mention that the sheer complexity of the reasons WHY Lelouch's problems took place definitely dwarfs a lot of Light's "Oops, I was too arrogant and did something stupid to make people suspicious" type of problems.
And you say that without the army he had, Lelouch would've failed. Well obviously that's the case, but that doesn't really change anything. The situations Light and Lelouch had were so different from each other that it's ludicrous to use the idea that Light requiring less people on his side meant that he's "more competent" or "superior" as a result. In Lelouch's shoes, Light would've had to do the same thing, and if Lelouch were in the Death Note world, obviously he wouldn't need an army to bring about what he was hoping for. Lelouch getting an army was simply because of his charisma and his ability to manipulate Geass to convince people to join his cause. It's no different than Light's persuasion abilities to avoid Rem killing him immediately and him getting power with the Kira Investigation members later on.
