It would be risky for Seattle to try approval voting as it hasn’t been used in practice very often. It appears just as shady as the plurality system it sought to replace in cases where it has been implemented.
While ranked-choice voting has been tested, contested, and implemented in multiple places, approval voting has a very low rate of implementation. While rank-chocie voting has been upheld through various legal challenges, approval voting has not yet been scrutinized by the American justice system. Thus, the validity of approval voting has not been called to question in any circumstances.
For more than 100 years, democracies from all around the world have used ranked-choice voting very successfully. It must also be noted that hundreds of important associations and parties in the private sector have used the method to elect their leaders. Seattle voters will choose the candidate with the most support and the fewest invalid votes if they choose this system for their primary elections.
By employing ranked-choice voting methods, voters might avoid feeling bad about supporting candidates from minor parties and third parties. In any case, ranked-choice voting provides for open voting, where participants don't have to worry about whether or not their vote will support their less-preferred candidate or whether or not it will be necessary to be unduly strategic.
Although approval voting may appear straightforward at first, it actually needs voters to perform much more intricate strategic calculations that are mostly based on minute differences in polling. Or, in the event of absentee voting, voters must try to decipher some of the tactical repercussions of their decisions amidst the candidates' and campaigns' advertising.
Ranked-choice is the only way for Seattle to successfully move away from plurality voting, a system that has been tested time and time again and proves to be an effective method at electing officials.