Wow this thread was a fun read!
I regret I am so late to the discussion but it was so interesting.
I hope the set up for the Zheihander system went well.
In short the way I felt about the differences is:
Psionics is powered by the character. More capable on the Internal
Magic is powered by something else. More capable on the External
As this goes into my view of the differences, I will frame the statement that at my table’s game, there is no magic/psionic transparency. They are treated differently, and unless explicitly stated, magics do not affect psionics and vice versa. So a Dispel Magic does not work on a psionic effect, and Detect Psionics does not pick up magic.
Magic broke the world of Athas. Both divine and arcane magic have had catastrophic consequences on the world. Its not that Psionics cannot. The Order could be believed to have that on lock down, preventing anyone from causing catastrophic problems with Epic Psionics. However, in the eyes of the multitudes, Magic is dangerous.
Unless you are educated in magic or know about spellcraft, it may be hard to know if your priest is an arcane spell caster. So that idea of ignorance is important in part. The lack of knowledge is rampant in Dark Sun. The ability to disguise spellcasting as psionics is also another way a spellcaster can try to hide. Its not that a Psionicist can’t do the same damage as a Spellcaster, but its because there is such a taboo against wizards that it can cause a panic and painful response.
However, the question about “how to handle psionics” is that in my understanding and works, is that it comes from the person. All creatures on Athas would have some latent Psionic potential, whether or not it is worked is another thing. With few exceptions, Psionics are limited in what a person can learn and gain. Powers can be basically written into a person’s mind, and reality warping powers potentially at their fingertips. Even time travel is in their reach.
What is important for the mechanics to model? Magic relies on an external source.
For Divine it is adherence to an external power. These powers are alien, and do not adhere to the same rules of reality as the common man. They may have grudges, logic, or rules that are hard to fathom. Breaking those rules can get someone cut off from their power source.
For Arcane it is a scientific manipulation of the world around a person. Arcane Magic draws on innate power, without permission, to cast spells. There is more power the greater fuel used, giving way to how Defiling burns more of the power to cast stronger spells. However, Preservation magic only uses enough to get the effect done. In a straight spellcasting fight, Defilers have the edge, because they are able to pull more power.
What can you do with magic that can’t be done with psionics?
Technically speaking, very little. A character can do much the same things. However, there are some very key and important differences. A character with enough money and influence can get access to any power, assuming they can manifest the power in the first place. They can have it put into their mind, or put a hand on another person and use a power from their fellow.
In combat, a person can get killed by an Energy Ball (psionic) just as well as a Fireball (arcane), but the Energy Ball can change its elemental type and does not need to be prepared. If you know it, you know it. However, Psions only know a certain number of powers. Outside of that power and influence mentioned above, they will never have the chance to learn things forbidden from their path. Though what they can do on that path is very strong, it is not as available.
A lot of powers for Psionics are focused on the self. They are not good for buffing other players. Their potential for self-buffs are immense, even something as healing another player is tricky. So your Psions are generally not viewed as healers. They can be functional and effective, but you’re probably not going to hire one to be a medic, heal your sick children, or provide protections against the elements. There are ways, but Psions are rather limited, and if someone did not spend their life and their potential to “grow” a certain way, they will not be available (EG: flight in general is difficult for Psionics, but not impossible, where as Fly is available for arcane casters and some air clerics).
Magic on the other hand, is an external source. They can prepare their abilities. Divine casters can request new spells, fitting the situation, bring back the dead, create food and water (that stays), and cure diseases. Arcane casters can study a problem and research a solution, developing a new spell, or find a hidden spell scroll and incorporate greater knowledge. Where as a Psionicist could theoretically do everything a Magic caster can, for Psions that’s a life decision. For a caster, its a day of preparation.
What criteria should we use to judge whether a game system has done a good job of representing Athasian magic and psionics and the fact they are different?
What is the cost of magic? That is the big decisive factor. Magic changes or breaks the rules. Whether it is adherence to an external force like a Sorcerer King or pulling energy from the world around you for arcane magic, there is a cost. The more powerful the magic, the more powerful the cost. However, you can do incredible things with that power.
Psionics is generated power from a creature. It can be stored, shaped, destroyed, but it is the renewable resource of Athas. The downside is it is far less malleable. You can become a powerful juggernaut and destructive force of nature. However, sacrificing life is a potent and horrific mechanic magic is capable of to do incredible things beyond the reach of a singular person. Even the Dragon needed slaves to sacrifice for Rajaat’s prison.