The difference is usually in the limit of detection. Generally, fluorimetric assays will be more sensitive. What sort of reader you have may also influence your choice.

Hello, fluorometric assays are a little more sensitive than colorimetric assays. Some labs will combine both techniques by using a clear bottom plate.  This allows for measuring a fluorescent probe for one analyte and a colorimetric probes for a second analyte.

I do agree with Karen A. Darbinyan Nick Gee