[Voralphion]

Since Frodo inherited the ring from Bilbo and he had shown some power to resist its influence on the trip to Rivendell, the council, well Elrond and Gandalf at least, decided that Frodo would be the best bearer of the ring. Whether Sam would have resisted its power longer and better than Frodo is not known. Sure he rejected the images it put into his head when he bore it, but if he had had the ring as long as Frodo had at that stage, maybe he would have been worse effected.
Also I doubt that Sam, even if he had been a more suitable bearer would have been able to throw it in the cracks of doom either, as the power of the ring ther was to great.