Compare and contrast Portia of The Merchant of Venice with Viola of Twelfth Night.

The big similarity is that both Portia and Viola disguise themselves as men at some point in the story. Their reasons for doing so are different, however: Portia does so to rescue Antonio from Shylock's vengeance, while Viola is disguising herself in a foreign land where as a woman she might be vulnerable to physical assault or others who would take advantage of women. She is also hoping to gain employment in the court of Duke Orsino, who might not wish to employ a woman.
As characters, the two contrast greatly. Portia comes off as more assertive than the gentle Viola. Portia manipulates situations to get what she wants, such as guiding Bassanio to choose the correct casket so she can have him for her husband. Viola pines for Orsino by contrast, though to be fair, she must hide her identity while working for him.
Portia has a sharp wit, while Viola's use of language is more lyrical. Portia is quite intellectual, considering how she's able to argue for Antonio's life successfully in court. Viola's intelligence belongs more to the realm of the heart, since compared to all the other foolish and lovesick characters in Twelfth Night, her love alone is pure and noble. She is wise when it comes to how romantic love operates, though she is as subject to its thrall as the other characters are.
http://www.shakespeare-online.com/plays/characters/sisterhood/sisterhoodportiamer.html

http://www.shakespeare-online.com/plays/characters/violabio.html

One of the main similarities between Portia of The Merchant of Venice and Viola of Twelfth Night is that both characters disguise themselves as men to gain access to spheres in society formerly forbidden to women. For instance, Portia disguises herself as a male lawyer in order to gain admittance to the court, successfully argue in defense of Antonio, and save the day. Along the same lines, Viola disguises herself as a boy in order to gain employment with Orsino, Duke of Illyria. As such, both women use male disguises to their own advantage.
One major difference between the two women is the manner in which they find love. While Portia enjoys the direct attention of Bassanio (not to mention several other suitors, who aren't so lucky), Viola spends most of the play ignored by Orsino, who is in love with Olivia and doesn't notice Viola until the end of the play. As such, Portia is able to find love relatively early in the play, while Viola must wait much longer to win Orsino's love.