What themes can I use to compare and contrast The Grapes of Wrath and Bud, not Buddy?
These are two great texts to compare! If I were you, I would want to talk about the central theme of the journey and how both texts place their protagonists on a journey during the course of the novel. The Joad family, for example, equally trek an external and internal path as the novel progresses. What is important to realise is that as they face setback after setback and disappointment after disappointment, their suffering is something that helps them develop and mature spiritually. Note how Ma comments on the change that has occurred in them as they develop this new outlook on life:
Use' ta be the fambly was fust. It ain't so now. It's anybody.
All of the four Joads show this new consciousness in action by the end of the novel by how they dedicate their lives to helping others, as Tom works organising migrants, Ma is forced to acknowledge that others have a claim on her time and energies, Rose of Sharon sees how her loss can allow her to give in other ways to the suffering around her and finally Casy works as a strike leader and organiser in the union, understanding that he has to work to live out his beliefs about life. Therefore the novel contains a definite internal journey that moves the Joad family towards a greater understanding of spiritual identity and what it means to be human.
In the same way, Bud in Bud, Not Buddy goes on his own journey to discover himself. He ostensibly is searching for his father, but the reader can easily identify that he is actually going on a journey as part of his coming of age, and the adventures that he has and the sufferings that he endures and triumph over point towards his greater understanding of who he is as an individual and where he fits in to the world. The fact that the novel ends by his finding of a home where he can be secure and loved and where he can have a purpose cements this theme.