I go with Lelouch.
Lelouch is practically smarter than Light in my opinion. He shows excellent real world solutions and tactics, even if the enemy tacticians find it odd or don't understand it many of the times. He also recovers from setbacks extremely well. His mind is like water, he keeps adapting every win or loss. I don't think Light's feats can match Lelouch's in practical thinking or adaptability.
Lelouch also is way better at organizing under stress. He kept an underground militia secret and organized, while Light attempted to branch out, and fell apart with only himself and two others running the show due to mistrust and disorganization, leading to a mistake.
Light will smoke Lelouch in any sort of test, IQ, or logic puzzle though. Light would also make a better detective than Lelouch, however, I do think Lelouch would not be bad, he just has a different intellectual skillset than Light for sure. But a more valuable one I would argue