Well, this is difficult. Sam is my favourite character but I believe in one way Sam is stronger than Frodo and in an other way he isn't.
I don't doubt that Sam would have been able to throw the ring in the fire of Mount Doom. I do believe in that way he is stronger than Frodo and the Ring wouldn't have that much power over him. Exactly because like Prog said, Sam is more "simple". That's also the reason, why he is the only one of the fellowship who comes unchanged out of this adventure.
But Sam's weakness is that he doesn't think very high of himself. He believes he's clumpsy and not so bright. So I wonder if he would ever made it to Mount Doom. Certainly not alone. He would need someone with him.
Frodo had much more selfrespect and selfestimation and even he wouldn't have got to Mount Doom without Sam. Not only because Sam protected him, saved him from Shelob and the Orcs and carried him when he had no strength left, but also because Sam kept Frodo from losing his mind, from going mad. There Sam proves again that he's strong.
So my point is, Sam would have needed someone too.
But I think his lack of selfrespect would cause him to fail. Imagine the scene when Boromir tries to take the Ring from Frodo. Suppose Sam is standing there alone, unsure of himself, doubting if he would ever succeed (like Frodo also did). Then Boromir shows up. He didn't use violence at once. He first tried to talk Frodo out of destroying the Ring. What would happen if Boromir would tell Sam that he feels sorry for him, bearing such a heavy burden. Asking Sam to give him the Ring, so he would be relieved. That it would be better to use the Ring in stead of destroying it. How would Sam react? What would he think? That such a big and important warrior like Boromir would know better that this simple Hobbit? Would that be in his mind so he would hand over the Ring? Or would he have the strength to remember Gandalf's words? Would he remember his promise to Gandalf? But I am convinced if Boromir would try to take the Ring by force, Sam would never give it up.
But then there is a second difficulty: would he decide to leave the fellowship? I don't think he would. I don't think Sam would ever decide to leave them all behind and sneak out alone. He's too loyal to do that. Here he would need someone like Frodo to convince him to separate from the group and continue by just the two of them.
Well, those are just a few of my thoughts. :)