The Incredible Hulk is extremely durable; but, more importantly than the sheer toughness of his hide, is his ability to recover from attacks by other superhumans. His healing factor is so good that it actually leaks into the realm of regenerative – Hulk can grow back lost limbs (all, presumably, except his head – but good luck separating that from his shoulders).
Furthermore, he can fight nonstop almost indefinitely – except when going up against a similarly powerful being. Then, the blows he takes are so massive that even he appears to slow down some; but his opponent had better beware – Hulk is always healing, even if it takes longer for him to gather himself.
The Man of Steel, on the other hand, is very, very, very difficult to hurt in the first place. In addition to Superman’s world-crushing strength, he has a level of invulnerability that is almost impossible to match for all but gods.
Although he doesn’t heal like the Incredible Hulk, the Man from Krypton takes damage with the best of them, often shrugging off the most powerful of superhuman blows and eating grenades to contain the damage.
He can heal much better and faster than any human with the aid of the Sun, and his vast reserves of energy are also due to the yellow star. This means his stamina is also almost illimitable, unless someone can tax him to the point that he expends it all and is confined to the hemisphere where nightfall reigns.
What is certain is that Superman versus Hulk would be a heck of a battle, and you can read about the full scope of their theoretical fight from a fan’s perspective by clicking here.