The talent description says that you may choose a number of Psychic Powers and/or Sorcery Arcana equal to Willpower Bonus.

Can these be different each time the talent is activated or do you choose them the first time and are stuck with them from then on?

You mean the "Psy Rating" Talent? In that case you choose once, when you take the talent, and are stuck with those powers until death do you part

No I mean the Power of the Warp talent from the Radical's Handbook, which is available to those who select the Daemon Vessel elite advance package.

Aha. The way I read it, you choose new powers each time you activate the Talent.

I don't think thats overpowered, as you'll not be using this talent many times