Not only is Washington the better team, but the Mystics are home and in a good spot. 
Connecticut breezed past Los Angeles in its semifinal playoff series. Not only weren't the Sun challenged by the Sparks, but they haven't played in a week. That's too long between games.
The Mystics eliminated Las Vegas, who was far more competitive than Los Angeles was, in their semifinal playoff series. The Mystics won that series on Tuesday. So they have the proper rest, but not enough days off to be rusty like the Sun figure to be. 
Washington led the WNBA in points per game at 89.3, field goal percentage and free throw percentage. The Sun can't match those numbers. Nor can they match up to league MVP Elena Delle Donne. As good as Delle Donne is, the Mystics have become a great team because of Emma Meesseman, who has become a star after not playing in the WNBA last year. 
The Mystics buried the Sun when they previously hosted them winning, 101-59. Connecticut is a bad road team. The Sun are 4-11-1 ATS during their last 16 away contests.
Washington is 7-2-1 ATS in its past 10 home games. The Mystics also are 13-6 ATS the past 19 times when playing above .500 foes.