You present a tough choice. I couldn't really decide between those two, except for the fact that most DMs I've played with hate Psionics, because they don't know enough about them. I've read the Psionicists Handbook (2nd edition, not 3rd edition.), and the way psionics are supposed to be played is a little different and bassackwards from the other classes. I can't remember the way psionic dice rolls were made, but I remember something about all devotions were done with an ability score check, but unlike any other check, it was NOT a critical success if you rolled a 1, but it was actually better if you rolled your exact ability score for the check. I've always wanted to try playing a psionicist, but I've never really understood how one should be played. All the other classes have some sort of image you can use as a reference as to how a character should act, but there's no archetypical (That is a word, right?) psionicist.
As far as which is the better
class,well, that depends on the scenario. I think psionicists benefit more if you travelled out of the Prime Material plane. Wizard spells get all jacked-up once you're in the Inner/Outer planes, but I haven't found anything in Planescape that affects Psionicists. Since psionics comes from within, I don't see how the planes would affect it. Let's say you were in Ravenloft, on the other hand. Ravenloft still has some altered wizard spells, but I think psionicists have a higher potential for misery in the demiplane of dread, especially when there's a roll of 20 on a devotion check. The horror characters are supposed to experience in Ravenloft can screw up their minds permanently (if roleplayed correctly!). A psionicist's mind can suddenly become his worst enemy.
Personally, I've come to prefer warriors and rogues, because I like high dexterity and weapon masteries (A character becomes
quite dangerous when you put every weapon proficiency he has into one weapon! Damn 3rd edition for changing the system from proficiencies to skills and feats! Grrrr!
Oh well. I guess it had to be done, because combat was
WAY too easy in 2nd edition.), and those character classes have a very wide range of archetypes to fit. Most wizards are just mysterious, clandestine, annoying, or a combination of the three.
Well, getting back to the subject at hand, I think I like wizards more, but not at early levels. A level 1 wizard is virtually worthless, unless he's got a helluva lotta charisma under his belt, because he's probably gonna have to talk his way out of a lot of combat (Keep in mind, there's a lot more to AD&D than HACK-AND-SLASH!). Overall, I think straight wizards are best left to the DM as NPCs. Wow, I've been rambling for a while, now! Getting me started on AD&D is a
dangerous thing to do!