Gandalf himself hints at it when speaking of Bilbo:

Frodo: “What a pity that Bilbo did not stab that vile creature, when he had a chance!'”

Gandalf: “Pity? It was Pity that stayed his hand. Pity, and Mercy: not to strike without need. And he has been well rewarded, Frodo. Be sure that he took so little hurt from the evil, and escaped in the end, because he began his ownership of the Ring so. With Pity.”

So it seems that your motives when dealing with the Ring can moderate the effect the Ring has on you. Obviously Frodo's and Sam's motives were fair which explains the difference in the effects.

Gandalf himself hints at it when speaking of Bilbo:

Frodo: “What a pity that Bilbo did not stab that vile creature, when he had a chance!'”

Gandalf: “Pity? It was Pity that stayed his hand. Pity, and Mercy: not to strike without need. And he has been well rewarded, Frodo. Be sure that he took so little hurt from the evil, and escaped in the end, because he began his ownership of the Ring so. With Pity.”

So it seems that your motives when dealing with the Ring can moderate the effect the Ring has on you. Obviously Frodo's and Sam's motives were fair which explains the difference in the effects.

Edit: There is a section on the effect of the Ring and its relation to individual's character and motives in a study by David Harvey - "ONE RING TO RULE THEM ALL - Study of the History, Symbolism and Meaning of the One Ring in J.R.R. Tolkien's Middle-earth".