These were the two best teams in the WNBA this season so it's only fitting they meet in the championship series. But I believe Seattle has more going.
The Storm have far more championship series experience than the Aces. Most of their players were on Seattle's 2018 WNBA championship squad. Las Vegas doesn't have that big-game pedigree. 
Seattle looked great in its last two games in sweeping its semifinal series against Minnesota that concluded this past Sunday. Las Vegas, on the other hand, had a full five-game grind-out semifinal series against Connecticut that wasn't decided until this past Tuesday with the Aces nipping the Sun, 66-63. The Aces trailed 2-1 in that series. 
The Aces suffered a key injury during that semifinal series losing Sixth Woman of the Year award winner Dearica Hamby with a knee injury. She's one of Las Vegas' top defenders. The Aces can only beat Seattle, the league's No. 2 scoring team, by playing outstanding defense. 
Seattle bench player Sami Whitcomb left the bubble so she's out of the series, too. But Hamby's loss is much more severe for Las Vegas. The Aces received no points from their bench in their last game against the Sun. Their three reserves combined to produce zero points and one assist while committing six turnovers and five fouls in a combined 24 minutes. Hamby averaged 25 minutes in the playoffs with a season average of 13 points and seven rebounds per game. 
The Storm have covered 23 of the last 34 times they've been favored. I believe they are the better team - as does the linemaker - and the situation is favorable to them.