What coil

What coil do u guys think it the best coil, I live in PA and I have the X-Terrac705 I was looking at the Minelab 10.5" DD Round Search Coil (X-Terra Series), but can someone tell me the difference between 7.5khc and 18khc, I know the 18khc is more gold sensitive but can it also pick up silver the same and coins the same? Thanks guys