So I just rolled 2 renewed coldies and got +15dmg and -9res, I was wondering what you guys say would add more effective damage for a frozen orb sorc.
My instincts scream -9 enemy res but you guys usually know better.

-res is almost always stronger. Your instincts are right. That's why cold mastery is WAY better than light and fire mastery.

On Frozen Orb, much of it depends on how many points you are putting into Cold Mastery. Cold Sorc builds are the general exception to the -res>%damage rule, as there are very few sources of increased %cold damage available.

If you are using sunder so frozen orb is your main/only skill, cold mastery should already be maxxed. Sunder breaks immunities and sets the enemy cold resist to 95%, so if your Cold Mastery gives -195cr, further -enemy res is pointless due to the minimum -100 cap.

So unless you are like a Frozen Orb/fireball sorceress or some other hybrid, CD%>-res on Sunder.