Now, low frequencies will penetrate the ground deeper and are more responsable to higher conductives as silver. The higher freq's will be better for the lower conductives as gold, and they are more sensitive to smaller objects. The real GOLD machines run at levels from 40+ kHz to 70+ kHz. But they're not that deep.
Personally I think there is no best freqency.
The beach machines of detector pro run at 2 kHz and they do very good in the wet salty beaches. But I don't know how they are on small gold jewelry.
The CZ-70 Pro i.e. is a dual as the DFX is, too. Both use the 3 kHz & 15kHz. Personally I like the performance better of the CZ. All the settings the DFX has I have no need for. But I'm going to divagate.
The best detector is that one you can handle with ease and that finds you the good stuff. IMHO