If a divorce is filed in one state but the non-filing spouse was not served, if they then file in their state, which filing takes precedence?

It is usually the first in time.

Generally, a lawsuit and eventual judgment is not valid without proper service on the defendant.  Furthermore, in most state, the court will not permit 2 actions to be going on simultaneously in two different states.  The reason for that is that courts do not want to have inconsistent rulings and waste time and inconvenience witnesses.  Indeed, the first case that is filed generally takes precedence over the subsequent action that is filed.  Many states call this the prior pending action doctrine.  Under this doctrine, the second case will be dismissed or stayed pending the outcome of the first case to thye extent that it resolves all the issues.  If you filed first and servced your husband, that case will take  precedence, presuming that CA is a valid place for the action.