Just from watching Talk Bass, it does appear that the FS and FX sets are the least popular, not sure why.
I have used the FS set and liked them quite a bit. They are similar in feel to the FL set; maybe people like the lower stiffness.
I haven't seen a comparison between the FM and M sets; I would think they are very close, although the Jamerson M set seems to occupy a different world from the others.
