The main differences between fleur de sel and Maldon salts are following:
The main difference is that fleur de sel has a bright, oceanic and delicate moist flavour. This moisture makes fleur de sel linger in flavours longer, while Maldon salt is dry, and thus fleur de sel has a longer lifespan.
Maldon salt is more of all-purpose salt, while fleur de sel should be used only as a finishing touch to enhance your meals.
Due to its minerals, fleur de sel has a greyish colour, while Maldon salt is white like table salt. Fleur de sel is harvested by hands and undergoes no processing at all. Maldon salt harvesting process involves more industrial processes where salt is filtered and boiled. As a result, fleur de sel retains more minerals than Maldon salt.
Good fleur de sel sells more than 3-4 times the cost of Maldon salt.