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 :-)
Hmm, I find the artwork in SoS vs. CotG to be the same difference. Then again, my taste in art is typically less sophisticated when compared to many other board gamers. However, the fact that this is so subjective means it could be a good reason or not to the next person.
Speaking of art, SoS has new art for the starter deck and "always on" deck (Mystics, Apprentices, HI, Militia, Cultist). Aesthetics aside, the layout colors, and patterns of one set here may be more pleasing and distinguishable compared to another.
Key (x/y faction/noun) x is the cost, or power of monster. Y is the honor it gives faction is enlightened, void, etc. Noun is hero, construct, or monster
As for the cards and the games themselves, in some ways, SoS is a superset of CotG. OTOH, there are some cards that round out SoS nicely. It's a hassle to deal with twice as many cards, but it can be worth it. For example,
-The Pen/Umbral Edges (6/3 v/c) can be a bit overpowered with just SoS due to higher odds of getting both of them. Adding another set keeps it more diluted and less of a power creep
-Two-Fold Askara (4/2 e/h) lets you copy the effect of any here, and is always fun to have around.