The lower frequency will detect hard metals (copper/silver) deepest, along with larger or thicker metal items deeper.
The higher frequency will detect softer metals (gold or nickel) and smaller thinner metals deeper.
Run dual frequency whenever you can and use Best Data. You'll get the best of all worlds. 15Khz is less prone to EMI and Correlate is good for high mineralization or in the midst of ferrous junk.
