+light dmg just adds more %ed to light mastery instead of increasing your speed overall damage. -light res decreases monsters resistances which effectively increases your damage by that amount.
For example:
Your base nova damage is 100
Your light mastery is 100%
Total Nova dmg =200
Now with +10% light dmg
100(base dmg) x110% (combined mastery and +light dmg)= 210 dmg
Now with -res
100(base dmg)x100%= 200
200x10% (extra dmg from decreasing enemy res)= 220 dmg
So in this example the -res lead twice the increased amount of damage from 10 to 20
Wow, thanks a lot man. I didn't know +light dmg just adds more %ed to light mastery. I thought it applied to overall damage. So better get -10% light res renewed sunder than +15%. You just made my day !