FFFg will burn faster which is, I think, an advantage with short barrels (pistols). Slower burning FFg is better for rifles and shotguns as the longer burning time builds up a longer, flatter pressure pulse which is effective further along inside the barrel. This takes better advantage of the longer barrel on a rifle. Although, to be honest, I'm not sure you could really consistently measure the difference except with some high quality (dollar) chronograph instrumentation...