Have you played an arcane caster before? If not then I would choose a sorcerer. Spell selection and casting is much easier, IMO, than for a wizard who must learn spells and prepare them each day. Considering the DnD Basic set that just came out WotC seems to feel the same way.
I am not really familiar with the Psion so I can't speak to that, but there are some things you might want to consider over all: the make up of the party, the style of play, the nature of the campaign setting, and the amount of "downtime" in the campaign.
To me the wizard works best in a campaign with lots of downtime and a permenent or semipermanent base of opperations, where item creation feats and the chance to learn more spells are optimized. Personally, I would always choose a sorcerer unless I planned on creating magic items, in which case I might choose an Artificer from Eberron or a Wizard depending on how focused I wanted to be. I just like the sorcerer better, it has a better magical feel to me.
If your DM allows it I would use the Monte Cook sorcerer from The Complete Book of Eldritch Might or the Battle Sorcerer variant from Unearthed Arcana, be a human, and take a bloodline feat from Dragon 311 or 325 (unashamed plug).