At the beginning stage of this epic quest, Frodo and Sam seem like highly contrasting characters. Frodo is heir to a fortune (by Hobbit standards), and Sam is his gardener. Frodo is well-educated and can speak some elvish, while Sam is a master only of comic songs. Frodo is set toward a high and lonely destiny as Ring-bearer, while Sam is mainly his sidekick and comic relief. Each of their distinctive qualities is highlighted even more through contrast with the other Hobbit.
So we have one good Wizard and one bad Wizard; but the main difference between Gandalf and Saruman is one of pride. Gandalf falls briefly into Saruman's trap because he has gone to him for help. Gandalf frequently takes counsel with other characters, including Elrond and Aragorn. His willingness to admit that he might need help is one of the characteristics that make Gandalf a force for good instead of evil. Saruman, on the other hand, has been too proud to ask anyone for help in his researches on Sauron and the Ring. This is what leaves him vulnerable to Sauron's temptations, and he finally falls to the Dark Lord.