With TTF the audio will be high pitched when ferrous numbers are below 17 and low for high ferrous targets like nails (17-35Fe). Since most beaches have little iron trash, the audio will be mostly high pitched.
On the other hand, TTC will give a high audio tone when conductive numbers are above 25. That would be helpful on pennies, dimes, quarters, and silver coins, and most large rings. Small, thin rings will likely give a low tone and the TID will show them in the low ferrous and low conductive region around 12Fe or somewhat lower. In TTF these same ring targets would give a high tone, as would bobby pins, foil, pins, broken sunglasses, a spoon, etc.
So, ultimately it depends on the target your most desiring to be alerted to. High tone on most coins and big rings - use TTC. High tone on most thin rings and nickels - use TTF. More important will be the Disc'd areas you set - you will not get an audio tone so be careful. You can overdo and miss those low conductive rings.