Both young women approach life from entirely different directions.
Olivia seemed to enjoy her self denial. She is self indulgent. One only has to look at Feste in Act I, scene 5. He proved that she was acting foolishly. When he asked her why she was in mourning, she replied it was for her brother's death to which Feste responsed that his soul must then be in hell. Olivia answered that it was in heaven. Feste replied that it was foolish to mourn someone whose soul is in heaven.
When she "fell in love" with the disguised Viola, it was more the novelty of new face. Her marriage to Sebastian was based on a misconception. She really didn't know him.
Viola, on the other hand, embraced life and, despite the fact that she thought Sebastin had died in the shipwreck, she decided to disguise herself as a young men and dress like her brother in order to pull it off.
She also recognized that Orsino is in love with the idea of being in love and in Act II, scene 4, she taught him about love. Their marriage was much more realistic since Orsino was already, in a sense, in love with her as Cesario.
Both women are educated and come from noble families but each have different ideas about life and love.