DH-RTv2: Difference between astropath and sanctioned psyker?

By AshesFall, in Dark Heresy Rules Questions

I'm a bit confused about the DH2/RT v2 fan-supplement distinction between Sanctioned Psykers and Astropaths, rules wise.

Character creation;

For A "Sactioner Psyker"
- Choose the Adeptus Astra Telepathica background (Does not grant PSY rating or access to psychic powers)
- Choose Mystic career, gain the Psyker Elite advance for free (Granting PSY rating and access to psychic powers)

or for an astropath (RT V2 supplement)
- Choose the Adeptus Astra Telepathica or Adeptus Astronomica background (Does not grant PSY rating or access to psychic powers)
- Choose Astropath Transcendent career (granting PSY rating and access to psychic powers)
- Gains all astropathic psychic powers (general) from DH2nd ed "Enemies Beyond".

or for an alternate Astropath route (not using the RT v2 supplement)
- Get what is needed for "sanctioned psyker" above
- Buy the "astropath" elite advance in DH2nd ed "Enemies Beyond" supplement.

Question: Which psychic disciplines can an astropath choose? As far as I can see there are no limitations listed, can they get all of them just like a "normal" psyker?
If so, why would anyone get the "Mystic-Psyker" variant instead of the astropath variant? The astropath just gets more better pretty much.

Any help is appreciated :)