Well first of all , sorry for my english.
I play with psionics .. been wilders my preffered, and master a game with a sorcerer/psion multiclass and i can tell you that they are way different.
With psionics you have a complete diferent system, its more like a rest to recover inmense mana pool than a vancian sistem, and that alone with the mind flabour makes it worth for me, but if you get a little deeper...
Psionics have thinks like ,mind control , not the mage dominate, but thinks like ... you become miself (personality parasite),erase memory, manipulate senses , kill yourself (death urge, and this is so strong. ... see that big nasty monster that kill your party in 1 hit makking auto-crits to himself and nothing more ).
Then you have premonitions, mental barriers, protection agains domination ... a new tipe of healing were you get the wounds from your friends and then cure yourself, or even send your wounds to your enemys. Yo create links in the "souls " of your enemys making that kill one will kill both...
On other think a mage summon monsters, psionics dont, they create thinks from their mind ...
Well if you look at the power list you will see the point. it has a different flavour, and if you look at the disciplines , the equivalent of a necromancer and a conjurer in psionics is more like , an invoker and a metamorfosing warrior.
But its hard to see the differences without playing it , and i dont know how deep it will be in the game.