This would be a real toughy. A military strategist prodigy vs. a detective the likes of which have never been seen. (For me Death note ended after volume 6, then it started sucking) I would have to say this would be an all out tie in terms of intelligence.
Lelouch has military power however. Never underestimate the information gathering abilities of a militarized force. Lelouche also has respect. L hides, making people not trust him. However, the way L works (As a detective) would be to find out Lelouche's (Zero's) identity. I am perfectly certain L has the ability to do that. But apprehending a military mastermind like Lulu would be troublesome. However Lulu's identity would be out. This gives L an advantage. He could then track down the main hanchos of the black knights based on their close relationships. In the end, I think L would win simply because he has the element of being able to find out information. Lulu has the ability to apply it more.
In a military conflict Lulu would win, simply because He has experience with such things.