Both protagonists exhibit perfervid individualism--a nonconformist, go-it-alone, alienated worldview. Meursault and Siddhartha reject mainstream religious doctrine and dogma in favor of lives devoted to simple pleasures. Both go through phases where they enjoy the company of women (Siddhartha eventually abandons this). Meursault loves the ocean, while Siddhartha finds peace in the river. In both, water is a symbol for life. Both are bored by the ennui of work and see the emptiness in materialism.
Siddhartha is much more actively on a religious quest than Meursault. Meursault is much more passive and static in his daily life (he hates trips). Obviously, the act of "murder" separates them: Meursault is more unhappy with society than Siddhartha and, therefore, lashes out in violence. Meursault relishes his role of anti-christ in part II of the novel, while Siddhartha is less of a rebel and outcast. While Siddhartha finds a reconciliation in the spiritual and physical while a ferryman, Meursault ultimately sees the world as a cruel, absurd place. Ultimately, The Stranger ends with a violent execution, while Siddhartha ends with nirvana by the river (as polar opposite as two novellas can be).