Which is a better propellant for nuclear engines, hydrogen H2 or Helium? Helium has higher molecular weight, but its monatomic so almost no Cp from internal vibrational states. I don't know how those factors trade off.
Of course if you can get the hydrogen hot enough to dissociate, then its the clear winner.
There is very little hydrogen dissociation until the temperature gets up around 3,000 K. I researched NERVA a couple years ago and I don't remember all my sources, but I'm pretty sure those engines operated at less than 3,000 K. We're therefore dealing predominately with molecular hydrogen.
It takes far more energy to heat hydrogen to a given temperature than it does to heat an equal mass of helium to the same temperature, by a factor of about 3 to 1. However, the materials of construction and the ability to cool the engine will limit the temperature at which the engine can operate. Let's say we run the engine at the highest temperature the materials will allow. This means that for a given reactor, we can put three times as much helium through the engine as hydrogen. Alternatively, we can heat the same mass of helium with a reactor 1/3 the size. However, at a given temperature hydrogen will have a much better specific impulse because its molecular weight is half that of helium.
So which is better, hydrogen or helium? The answer is, it depends. Hydrogen is a more efficient fuel that produces a higher specific impulse. On the other hand, helium makes more efficient use of the power plant. I would say that in most practical applications hydrogen is the better choice because you require less propellant, which is usually the most massive part of the system. If you had a situation where not much propellant was needed, helium might be better because you could downsize the reactor and reduce the dry mass of the system. However, in the latter case it's probably simpler to use chemical propulsion.