Can someone explain some of the advantages of dual frequency sonar, like the LMS-525C DF, vs single frequency sonar, like the LMS-520C? Is it worth the extra money for the DF unit?

Mostly see Dual Frequencys used in saltwater applications where max water depth is a big concern. Some of Lowrance's DF models get up to 3000' and most of the single frequencys sta around 900. JC