Since Ascension: Storm of Souls is both a standalone and expansion to Ascension: Chronicle of the Godslayer, is there any reason to buy COTG? SOS adds new mechanics and seems to have better artwork.
Not to be Captain Obvious, but you would buy CotG if you want the cards.
I quickly glanced at the card list and a few "must-haves" stuck out:
Enlightened: Master Dhartha, Twofold Askara, and Tablet of Time's Dawn (though you might NOT want the possibility of infinitely many turns...)
Mechana: all the constructs in this set are valuable, Burrower Mark II maybe not as much.
I own all of the Ascension blocks and personally play CotG the most since the minimal mechanics allow it to be more accessible to my wife. I find it as a good introduction into the world of Ascension and then once she becomes more comfortable with it, I'll pull out the other blocks :-)