In actual fact....I do not believe that Sam or Frodo ever put the Ring on while actually
in Mordor. I just read that part in the book, and he took it off as he entered... If he had it on at all, he put it on before entering Mordor proper and took it off after a couple steps within.
As for who was more resiliant? I don't know. Sam was childish and pure and perfectly simple. Frodo, on the otherhand, was not so childish. He had proper learning and higher, more elvish thoughts. They were both of them strong in their different ways. Sam was strong because there was not as much reason to be powerful or to rule...he had no wish to. Frodo was strong in another way...a more knowledgeable way, if that makes any sense.
I think, though, that Frodo was more cut out for the task than Sam was, and that was why Frodo got the job of carrying it to Mount Doom, and Sam got the job of taking care of Frodo on the way there.
-- Folwren