Siberys or Reabsorbtion

[Ruethgar]

Which is the better choice? Siberys grants +4 effective charisma for bonus spells and max spell level, reabsorbtion would let me take back a failed spell. I plan for the character to become a high level which leans me more toward reabsorbtion to at least partially mitigate spells lost to immunity and spell resistance, but bonus spells are always good.

[holywhippet]

I'd say Siberys. Unless a monster is something completely unknown to you, you should generally not be casting spells that won't work. D&D is a lot about action economy. Getting a spell back is poor compensation for not doing anything effective that combat round.