FY, by far, is a lot more brilliance and overall more decisive than LL. His mastery and extensive knowledge over Gu refinement would bring in a big advantage for him. The Gu system is going to be supremely challenging for LL to maintain and use. Hell, he might even locate LL and kill him before LL could do anything.
Also, the Immortal Gu can only be one in existence, so either FY have it or LL have it.