The OU is 21, so has to be 2 sets and no tiebreaker. Sabalenka actually beat Iga last year in finals in 3 sets 6-3 3-6 6-3
Iga has only lost 1 set this week. Sabalenka has lost 4 sets and tougher matches especially the semi finals against Rybakina
I think time on court might be a variable that will help Iga win.
Personally I am betting Iga -1.5 sets, so based on that I would also lean UNDER