Negligible at that range, for close range the difference is almost entirely cosmetic. Technically a good free float system is less fussy, and generally simplifies the upper. FF is good for modularity, you don't have to deal with a pinned fsb and the challenges that come with it when it comes time to replace parts. Generally a good free float system is a better platform for mounting accessories, again being more modular than the original clam shell handguard design.