FL its a legendary refractor but ED too... Fluorite or ED glass alone means nothing.
It is the pairing with the other glass that determines the final correction level (and not quality).
Exemple : FPL-53/BK-7 is less efficient than FPL-51/KZFS2.
You have to get back into the context of the time, Takahashi was already offering its Fluorite, to cope Vixen stopped the ED -S series to in turn offer Fluorite, which was more of a seller.
I was able to compare an FL 102 and an ED 102 together on Jupiter, hard to see any really difference.
Sure, Vixen Fluorite its a legendary premium telescope.