Being a Hulk doesn't automatically give a character psi defenses, or mystic ones. Banner is a troubled soul, and for some reason it's been translated into him having mutated psi/mystic defenses and abilities, such as being able to see astral beings. Rulk has yet to reveal these abilities.
Rulk may be stronger than Thanos, but this must still be proven. Thanos, has more weapons to use than mere brute strength and energy bombardment. I would hate to see the two of them battle, and the result of it looking like the Champion vs Thanos debacle.
It would/should be a good fight, but that's about it. Thanos should win this, due to power set.