How does Orsino treat Viola in comparison to how he treats Cesario?

Viola and Cesario are actually the same person in Twelfth Night; however, when Viola disguises herself as a man she becomes Cesario. In the play, we realize that Duke Orsino treats Viola (alias Cesario) differently according to whether he perceives Viola as a man or a woman.
Viola, disguised as Cesario, becomes Duke Orsino's attendant; the duke is impressed with the young page and decides to use "him" to help him woo Countess Olivia. In the meantime, however, Viola/Cesario falls in love with Orsino but can't confess her love, as the Duke sees her as a man.
Orsino is certainly fond of Cesario, but this never translates to love. However, when he finds out that Orsino is actually Viola, he suddenly reciprocates and chooses to marry her instead of pursuing Olivia, which had been his main love interest for most of the play. The Duke's feelings evolve from fondness to love when Viola's true identity is revealed. (Note: Some scholars make the case that Orsino is romantically interested in Cesario—he is already enamored with the disguised Viola and is freed of social restraints when he realizes he can legally marry her.)
Orsino treats both Viola and Cesario with respect and affection; the fact that Viola is actually a woman then amplifies these feelings, leading to Orsino and Viola's marriage. It is possible to argue that Cesario and Orsino shared a deeper bond than Olivia and Orsino ever did, but this bond could not turn into love because Viola was perceived as a man. Once Viola is seen as a woman, there is no impediment to love and Orsino is free to marry the woman with whom he has a true connection. In fact, Orsino's infatuation with Olivia appears extremely fickle, based purely on superficial aspects, and is immediately cast aside once Viola's true identity becomes clear.