Ugh. No psionics, please. I just got my head around ToB.
If you know your way around magic, psionics is actually easier.
The basic differences are as follows:
A.) There's no arcane spell failure, so you can cast freely in armor.
B.) Power points from different sources (race, different classes, etc) pool, unlike spell slots which are specific to class, and you cannot spend more power points on any one manifestation than your Manifester Level (so a 3rd level psion can only spend 3 pp on a single power). There are a few ways to temporarily increase your ML, but they tend to have consequences (such as taking damage from the Overchannel feat).
C.) Very few powers scale freely, so you have to pay additional pp to 'augment' a power (and most powers have specific effects you can spend power points on, which increase the effective DC and other effects).
D.) Instead of somatic, verbal, and material components, psionics has 'psionic displays' which alert creatures in the area to your position. You can suppress displays with a Concentration check.
E.) Psions, unlike wizards, must specialize in a psionic discipline, which is the psionic equivalent of a school of magic (though a variant in Complete Psionic overrides this). As a result, they gain additional class skills, and access to powers that are unique to their discipline. You can access powers from other disciplines (or from another class's power list) only if you spend time and XP on research, or if you take the Expanded Knowledge feat.
F.) And lastly, 'psionic focus'. You make a DC 20 Concentration check as a full-round action to gain focus (which is cut to a move action with a feat). By itself, all it does is allow you to 'expend your focus' (meaning you become unfocused) as a free action to 'take 15' on a Concentration check. However, you can get feats and class features which have specific effects if you're currently psionically focused, or if you expend your focus, such as metapsionics. Metapsionic feats work like metamagic, except you have to expend your psionic focus in order to activate one, and it's one focus per metapsionic feat (so unless you gain feats or class abilities that grant you extra foci, you can only ever use one at a time).
...And other than a few minutia here and there, that's it.
