Advise the Adviser

Adviser or Advisor???

AP style tells us to use adviser rather than advisor but it does not indicate why.  I am interested in learning the reason.  Perhaps it has to do with the etymology of the word.

Maybe because the root verb (“advise”) ends in the letter E, the -er suffix is preferred to the -or suffix.  An analogous case is the verb organize, which becomes organizer (and clearly not “organizor”).

Dictionaries say these words are synonymous.  Neither seem technically incorrect or correct.

What do you think?  Comment back – do you use advisor or adviser?