Why BH3 its classified as a "Molecular Hydride" and AlH3 its a "Intermediary Hydride" First, I understand that, on $\ce{BH3}$, they eletronegativity difference between $\ce{B}$ and $\ce{H}$ is very small. Which mean that the $\ce{B-H}$ bond is less polarized. 
But, I couldn't find a relation of this, to the fact that he is a Molecular hydride.
And, secondly, I couldn't understand why $\ce{AlH3}$ its "intermediary", and not molecular, as well...
Alane is not a molecule: https://en.wikipedia.org/wiki/Aluminium_hydride You can't isolate molecules easily because they polymerize.

Borane on the other hand is known to exist as a dimer: https://en.wikipedia.org/wiki/Diborane

The difference in bonding is likely due to "hypervalency" (or similar concept with different terminology) in aluminum.