Multi. If you go single frequency then you obviate the advantage of multi IQ which is what gives each mode its unique "personality". Multi IQ helps with ground compensation and target signal processing. Iron Bias is disabled in single, too. If you do want to go single, then the frequency choice is based on your target of interest. Also, in the case of the 600, in single frequency, you lose access to the 20 and 40 khz frequency components that you have in Multi that are key to exciting mid-conductive targets like brass, lead, (relics), small jewelry/targets and gold. So if that is your focus, 15 khz is the best alternative you have on the 600 for mid conductors. If you are going after silver or deeper/larger targets 5 khz. 10 khz if you want to split the difference. Note the lower frequencies will be more susceptible to EMI.