Is a cambered or symmetrical propeller airfoil more efficient?
Cambered airfoils are used on propellers as they create less drag for a given Cl than a similar symmetrical airfoil. 

The primary use for uncambered or symmetrical airfoils is on flight surfaces that must provide lift in both directions equally well, as in a vertical tail or aerobatic aircraft wing. Propellers only provide forward lift or thrust (ground reverse is too little used to matter) so camber works better.