Whichever one you can find for cheaper. And/or whichever looks better to you. Flourite is baked clay, Eco is volcanic rock, and both have similar CEC (nutrient absorption rates). Neither has any inherent nutrients, despite what the packaging may suggest. Cheaper high-CEC alternatives are Turface, Diamond Pro, and other baked clay alternatives used in farming and for baseball infields.
You can cap dirt with either one, and, if not, fert tabs will always help.