Cation Exchange Capacity represents the ability of the sand particles to hold nutrient positive ions, like Fe, Mg, Mn, Ca, NH4, etc., where they will be easier for plant roots to pick them up. This is a definite advantage that Flourite has over pool filter sand. Another sand that can be used is zeolite pool filter sand, which is sand made up of particles of zeolite, which is a mineral form that has an even higher CEC than Flourite. Unfortunately, I have only found 3 zeolite sand products, and two of them are very ugly as a substrate, the one I used being a sickly greenish color.
The problem is that while a high CEC is theoretically a big advantage for plant growth, I don't recall seeing experimental results showing that it is 100% better at growing plants, or 10% better, or 1% better.
I'm currently using plain quartz pool filter sand for my two tanks.
EDIT: I just remembered one test result I have seen. Tom Barr did some testing using two, as I recall, species of aquatic plants, with a few different types of substrates, among which were Lake Tahoe silt, soilmaster select, which is a high CEC material, ADA Aquasoil, S.F. Bay Delta silt, etc. He judged the results, again as I recall, by drying and weighing the plants that grew in the substrates. The soilmaster select was one of the best, Delta silt was one of the best, and Lake Tahoe silt was one of the worst (being largely quartz sand, I would assume). ADA Aquasoil wasn't the best! These results might suggest that high CEC materials do have a significant advantage over pool filter sand.