Tldr, whichever is higher.
Whenever you have multiple ways to calculate your AC, you choose which one to use.
So unarmored and with no shield? Whichever is higher of 13+Dex (Scales) or 10+Dex+Cha (Quick Witted).
If they're using a shield, they can still use the Scales' 13+Dex(+ shield) calculation