The main difference between spirits and wisps was the kind of energy that they were made of.
Even though they were both beings made of pure energy, that didn't mean that they were the same.
The spirits were made of pure elemental energy that occurred in nature. It could be said that the spirits were the children of the worlds themselves.
However, it was different when it came to wisps because unlike the spirits, the wisps weren't made of natural elemental energy found on worlds. Instead, the wisps were made of residual law energy.
Wisps were creatures that came into being when residual law energy that was strong enough separated from the being that they came from and took form of their own. This form that they took were as wisps, so they were considered beings of law instead of beings of elemental energy.