64 FATS are a slightly fatter wound version of the 64S set, which makes the FATS a bit punchier and hotter compares to the 64S. The response of the 64S FATS is beefier, and has more a wider mid-range tone when compared to the 64 set.
64S FATS are great for rock, country, and metal. While they have a large high-end range, it's still a versatile and fun pickup great for gigging, general play, or recording.