Seer, Shaper, Telepath, Egoist, Nomad, Kineticist which one?

[Sgtpepper]

If or when you play a psionic how do you decide which of the disciplines you choose? Do you go for damage? Usefulness? Or whichever one sounds cool?

[Urpriest]

Tinker, Tailor, Soldier, Spy!

In general, it's just like picking a school for a Wizard. Each school has its own themes. Want to summon or BFC? Go Shaper. Mind control and the like? Telepath. Blasting? Kineticist. Gishing or healing? Egoist. Info-gathering? Seer. Unique teleport-tricks? Nomad.

If you're going for a general "I want to replace a Wizard and be powerful" perspective, I'd advise Shaper, maybe Seer. The distinction roughly boils down to whether you'd go Focused Specialist Conjurer or Diviner as a Wizard.

[AmberVael]

I definitely don't go for damage. Basic psion really has enough powers that are decent for that, you don't need to specialize in it further, particularly since damage isn't really the best option anyway.

Generally, I try and get a compromise between potency and what best fits my build and concept. Seer is my favorite, because their discipline list is fantastic and it meshes well with the theme of psionic powers, and fantasy in general. Plus, it allows for a certain air of mystery which I find appealing. I like hoarding secrets, and Seer definitely helps with that.

[navar100]

Damage as a Kineticist works for Psions because Kineticists get to choose what energy type to use when they manifest the power. They aren't hampered by the various energy resistances and immunities monsters have because they can just use something else. In addition, not everything about the Kineticist is blasting anyway. You get to move objects, which can be quite handy, and even move creatures.

As it is, in 3E Discipline only really matters for the various Discipline only powers. If a particular Discipline has several powers you want, then that's what you should use. If there no power you really, really want, then don't take it. If there's only one or maybe two you really, really want, that's what Expanded Knowledge feat is for.

Pathfinder makes the choice a bit harder because you get class abilities depending on the Discipline chosen. Depending on your personal taste those abilities might be a factor on your choice.

[Gavinfoxx]

Spell to Power Erudite
Erudite
Shaper

in that order!

[13_CBS]

As others have said, Kineticists make for excellent blasters partly because you can choose energy types, but also because you get some delightful powers like Energy Missile. If you're going with the unnerfed version (so not the Complete Psionics version), then it's 3d6 + Xd6 (X = each additional power point you invest to augment the power) energy damage of your type. You can take advantage of this not only to overcome resistances, but also to get around "save to avoid all damage" defenses like Evasion. Remember; Cold energy "Energy ___" powers force Fortitude Saves, not Reflex Saves.

Unfortunately, unlike Magic Missile enemies do still get a save to halve the damage, but the power is excellent for hitting multiple targets without having to hurt your teammates as other Area-of-Effect powers would.

Control Body is also great for shenanigans, especially on skill-monkey types who might not have a lot of anti-psionic/anti-magic defenses and still have poor Fortitude saves.

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Shapers, on the other hand, are extremely flexible. Just their unique powers can blast (Hail of Crystals), Battlefield Control (Concealing Amorpha), Utility (Minor/Major Creation), Save-or-Suck (Ectoplasmic Cocoon), Save-or-Die (Crystallize), summon a Tank/Trap Springer/Scout/Flanker/Grappler/Blaster (Astral Construct). Play a Shaper if you want a Psion who has a tool for almost any situation just among his unique powers.

Oh, and you can play god with the Genesis power (make a demiplane).

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I don't have much experience with the other Psion disciplines, but one thing of note:

Egoists get direct access to Metamorphosis, aka Polymorph, aka one of the single most powerful things you can do with a Standard Action in the game. Grab the feat Metamorphic Transfer and you're likely to break a campaign just by accident. Combine that with Hustle (also an Egoist-unique power, though Psychic Warriors can also get it) to make yourself even more broken. Egoists can also do some minor buffing (Psychofeedback), restoration (Restore Extremity), and healing (Empathic Transfer).

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Fortunately, thanks to Expanded Knowledge and the Psion's bonus feats you can grab a couple of discipline-unique powers even if you're not of that discipline, so if you want the flexibility of the Shaper but still desire the pinpoint blasting of Energy Missile, you don't necessarily have to go Kineticist.

[sleepyphoenixx]

Some of the Psion ACFs are pretty nice. Nomad gets a personal dimensional pocket that can't be taken away. Telepath gains telepathy and thus access to mindsight.
Without those it doesn't matter too much. Shaper gets good discipline powers at most levels. Telepath has Schism. Seers get all those nifty divination powers.