You can go both ways with it. With the "one class with half-dozen subclasses" route, you can design an entirely new game mechanic for psionic abilities that is different than the spellcasting mechanic system. Which is good for those players who want psionics and spellcasting to actually be two completely separate things. The downside though is that you need to figure out how to make "warrior" psychics, "rogue-like" psychics, "monk-like" psychics, "mage-like" psions and so forth, all using a single class as a foundation. And the question then becomes how different can you truly make them if they are all using the same foundation? They all would have the same hit die size, they would all start with the same proficiencies, they would all have the same "psionic" slot mechanic that they'd have to try and layer upon for those subclasses that would use more "castable" disciplines as opposed to buffing weapon use.
As opposed to what Mike seems to currently be working on, which is letting a Psychic Warrior start with being a standard Fighter (getting all the warrior stuff a Fighter gets) and then layering psionic ability on it. Same way with a Soulknife getting the standard Monk stuff and then psionic ability on top of it. Psion the same way with the Wizard foundation. It allows you to get the foundational feeling of the particular psionic class with psychic ability layered on top, rather than one single class that you then have to try and create all these various offshoots that feel different from each other.
I really don't know what is better quite honestly. I will say that I actually thought the "one class to rule them all" methodology of the Mystic was quite clever, and that the game could do with an entire different subsystem for psionics than using the spellcasting slot / spell point foundation. But I could certainly understand that if they've been working on Dark Sun right now and trying to figure out how to integrate this new subsystem into the game so that you could have things like "wild talents" (which quite a number of Dark Sun characters *do* have)... if there was no good way to use their new psionic subsystem and "hand off" some abilities to the other classes in a wild talent sort of way, then making psionics like spells would probably allow it to work better (since we already have the Magic Initiate feat to mimic.) Psionics-as-spells are easier to integrate into the game than a new subsystem. But a new subsystem would make psionics truly different and compelling.
The fact that that they have tried (or are trying) both ways to see what is better tells us that they haven't unlocked the answer yet, but also that they want to make sure whatever they come up with is the right call. And goodness knows that if they release this new system as a UA that Mike's been working on during his Happy Fun Hours... we'll all get a chance to comment on which of the two directions we thought worked better. And there's really no rush, since I suspect the soonest we'll ever see Dark Sun (and psionics) would be the Fall of 2019... but even that's possibly unlikely. Hints have seemed to indicate them going one new campaign setting at a time, and that Eberron being the next one up. Which means they could be Fall of 2019, and thus pushing Dark Sun and psionics back to Fall 2020.