Not to be insulting, but are you sure you are viewing HDTV content when you make the comparison? If the content is "upconverted", 480p will look better than 1080i, as the increased resolution will detail artifacts in the upconvert.
Otherwise, my suggestions would be possibly your component cables, or calibration of your Samsung.
When I watch movies on HBO or Showtime HD, I see grainy picture. When I watch Discovery HD Theater, I do not. Could this be what you are seeing? If so, you are seeing the grain of the film with the increased resolution. Just a thought.