Light would win, hands down.
Lelouch is intelligent, but he's prideful, and that's his downfall. Light only succumbs to pride a few times (though the results have a big impact; see Lind L Taylor, Swimming Lessons, etc) while Lelouch has a fancy for the grandious.
Light would SO be able to infiltrate the Black Knights, rise to a top position because of his incredible deduction skills and soon be one of the top commanders in the army. Then he'd stage a scenario where through several means of manipulation he'd make Lelouch crap his pants in front of the British Empire (thus revealing his true identity, as Zero shits black), then he'd go "JUST AS FUCKING PLANNED" while sitting in the main base smirking like a bastard, yet not being noticed because everyone's looking at Zero's shit.