Choose 8KHz. 16KHz (or even 32KHz as some ESC's have) should only be reserved for incredibly incredibly low IR motors. 99% of motors out there run on 8KHz, and sadly if you set it too high you will end up with a dead motor and esc, high amp draws from packs and plenty of melted electronics followed by even more smoke
.
If they have timing settings in the ESC, now thats worth a tinker, but as for frequency, leave it low.
Ian
TrakPower