edit: disregard this. I was very wrong.
Damage-wise, purple Nightguard is 10% more efficient than Nox (11/power vs 10/power) and has 5 aoe.
Nox have 17% more healt per power (7.7/power vs 9/power) and counter Ice Guardian.
I would argue that purple nightguard are better than nox againt frost with the following statements:
Damage is more important than healt, because you want to kill as many MA as possible before your opponent cast Home Soil and Wintertide.
MA are often clumped together making 5 aoe quite useful.
Ice Guardian should be ignored, by microing your