Neither.
Both C# and F# are excellent languages that poses the capabilities to examine large data structures in an efficient manner. Virtually any solution coded up in one language could be coded up in the other. True there would be some syntactic differences and the occasional language construct which doesn't port cleanly between the two. But neither language has an inherent advantage in this area.
Instead of focusing on the language here I would focus on the algorithm. That much more than the language will determine the efficiency of your program.
