FAST requires ex-military aircraft. FFI was created "for the rest of us"
The programs are very similar. Going through the FFI program in RV's, you will fly in standard formations to aerobatic maneuvering limits. Many FAST programs do not train to that performance level.
FFI and FAST are equivalent in the eyes of the FAA and at airshows. The flight lead always has final say on the makeup of his or her flight. Some warbird formations require a type patch, which essentially signifies a checkride in the make and model.
Tim Redden
FFI Flight Lead