The main difference between the two providers is the fiber availability. Sparklight employs a fiber-coaxial network, which means coaxial lines that are traditionally used for cable TV have been adapted to deliver fiber internet service. Using these lines instead of the copper cables allows for higher bandwidth. Sparklight also offers TV and phone services as bundles.
CenturyLink offers DSL and fiber internet services and uses existing telephone infrastructures for the former. It has a strong presence in rural areas, providing consistent stable service, although fiber may not be offered in some areas. CenturyLink gives you the option to bundle your internet with phone service.