To get back to something, I'd like to explain a bit more... I have done a little picture (quite crappy, I know, but it works, I guess

) to show what I mean.
The picture shows how the power of the psion's (fully augmented) powers (blue) and the sorcerer's (fully scaled) spells (red) compare at 10th level, including their respective costs (PP / slots).
Sidenote: The comparison at 10th level does favor the sorcerer already, a fair comparison would be at 10th
and 11th level, where the sorcerer would surely lose some ground, lacking the 6th level spells!
The scale on the vertical axis is spell/power level times caster/manifester level (capped at 5 for 1st and 2nd level), which should roughly show the relative power level of a single spell/power. On the horizontal axis the number of spell slots or
fully augmented manifestations is put into relation.
What can be seen is, that the total area of blue and red are about even, which makes them roughly equal in terms of total power in a day. The psion has more powerful effects, while the sorcerer lasts longer.
The problem in this comparison, and which is also the problem of Sorren's damage comparison, is, that the sorcerer's advantage comes into play
only, if there are enough situations to actually use those spells, if the whole horizontal distance in the picture is used up, whereas the psion can make use of the full potential pretty much every day. Surely, this depends on playing style, but it's clear, that the psion is far more likely to bring the full potential to bear, since the sorcerer is required to get into more situations where spellcasting is needed to do so and this simply won't happen on every single day. If you cut out part of the picture from the right, you'll only remove from the sorcerer, not from the psion.
Furthermore, this is only one option for the psion. There are plenty options to stretch this total power out further. The psion can also try to mimick the sorcerer and manifest powers only so far, that they come even with the spells (which in most cases is less than the maximum, even for some 5th level powers). When doing this, however, the psion will not be be able to fully reach the sorcerer's potential. Roughly 75% is a reasonable estimate (which I have tested against the spells my 10th level sorceress has cast in the last gaming session, which was pretty much a full alotment of daily spells, using powers which would achieve the same or higher effect). This flexibility further makes sure, that the psion will be able to bring the full potential to bear, whereas the sorcerer is still limited to the slots and can only use them in exactly the way shown in the picture.
Now, how this applies to a specific group has a lot to do with playing style, as has been mentioned already. If a party has only few encounters, the psion wins hands down with no contest. If the party has multiple encounters, the sorcerer gets even, or exceeds the psion somewhat, as the psion will then be forced to go for a less aggressive approach, naturally.
Basically, this means, that to achieve balance between the way those two classes cast/manifest their spells/powers, the party has to be challenged with many encounters each day (to a degree, this is also true for spellcasters versus non-spellcasters, but the psion makes this even more extreme).
If you go back to
THIS post and look at the first half (roughly), the initial comparison is what I have tried to picture here. I think that at this point the psion and sorcerer are about equal, each with their own advantages and disadvantages.
However, if you then look at the second half ot the post (the +/- list), then you'll see why I think, the psion is a lot more powerful as the sorcerer, since that stuff has all not been included in the initial comparison (the flexibility (of PP distribution) has been mentioned here).
And that's why I don't see how these two classes can be balanced.
Comparing the psion with the wizard goes a similar way, tho many items on the list will then be irrelevant, as the wizard has them as well, however, the ability of the psion (as the sorcerer) to spontaneously manifest powers, coupled with a power list, which is
a lot closer to the content of a wizard's spellbook than the sorcerer's spells known, and the fact, that the wizard has fewer spells per day, will also leave the wizard behind.
Now this does not necessarily have to be only a problem with the psion. Many people consider the sorcerer underpowered. But in my experience, while they might be slightly behind the wizard, they surely are not
that much behind as they are obviously, when compared to the psion.
I hope my thoughts become a bit more clear this way.
Bye
Thanee