Illuminated Devotion Ascendancy

I wonder what happens if i take the illuminated devotion ascendary
without taking arcane blessing because i linked arcane surge to
a curse, 15-20 percent more (multiple) spell damage (depending on the
level of your arcane surge to trigger it) can be much better than 40
percent increased (addition) spell damage from the ascendary.

Does anybody know if both effects add or does one of them overwrite
the other or is the better one chosen?