It's fairly straight forward - the stronger the creature, the greater the value of each reward. In this sense, the "strength" of a creature is determined by all creature variables including all creature stats, damage, resists, abilities & AIs. In practical terms, any of the champions are going to give far more training points per reward than something like a rotting corpse.
We are still working on tweaking a training scheme that takes into consideration the fact that 1 slot pets cannot fight against higher level creatures and take advantage of the higher gains. As we continue testing and iterating we should have a solution for this. The feedback we've received thus far from those who have been so helpful on TC1 is incredibly useful, and we encourage everyone to hop on TC1 and check it out!