Almost no difference. Medium or less is rarely observable in most lighting conditions. Under black light (UV) a stone with fluorescence will glow. A lot of people don't mind, or like, or in some cases seek out fluorescence.
Levels:
Inert/No Fluorescence +
Faint/Slight Fluorescence +
Medium Fluorescence
Strong Fluorescence *
Very Strong or Extreme Fluorescence *
+ In January of 2003 AGS changed their grading of inert and faint fluorescence to "negligible."
* Strong or very strong fluor can have a visible effect. In some cases this can cause a milky or cloudy appearance, which is to be avoided (happens rarely). On the other hand, strong fluorescence can help a diamond of lower color appear closer to colorless.
(this all pertains to blue fluorescence by the way)
Edited to reflect AGS 2003.
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