Both. Choosing not to run for re-election. Basically giving his notice. He's quitting. Like Ritter... Only he doesn't have the "for my family" excuse. (Almost laughable for Ritter, during the previous election he was proud that he spent 16 hours a day in the DA's office.)
Well, not running for election isn't the same thing as quitting. Quitting is what Palin did, resigning from office.