Neither of them is a hero.
They are two halves of a single hero. (No, this is not a joke about being hobbit-sized.)
Frodo is the one who has to bear the mental torture and hardship of the Ring–but without Sam, he couldn’t have. Who is there to offer a joke, a smile, a hug, an unjudging ear, an extra bite of bread when Frodo looks peaky and ill, a doubled-up bedroll when it’s too cold for a small hobbit used to a good hole with a warm fire? Frodo would have failed long before he reached Mount Doom without Sam to cheer him on.
Likewise: Sam could never have made the journey alone. He doesn’t have Frodo’s mental strength. But he’s a gardener: his job is to make things grow, to nurture them and make them flourish. Who is there to give him a purpose as he travels across a land so big, so far, hoping to save his home?
Separate the pair of them, and neither would have made it. Frodo needed the shelter of Sam’s heart in order to find his way–and Sam needed a leader.
It’s literally love and friendship and camaraderie that save Middle Earth. Neither could be the chosen one alone.