I need quick help, what is the best shadow defence helm line?
Shadow defence helm.
I wrote a better guide: http://forums.spiralknights.com/en/node/78944
I also have an armor guide. The short version is: Asking for shadow defense is asking the wrong question. You should instead be asking about offensive bonuses, followed by status resistances.
As long as we're still here, let me point out that there's a Kataclysm going on. So this is your chance to get the 3-star Black Kat helmet, for a mere 70 Ancient Pages. There's your shadow helmet.
while we're on the subject, is it worth it to use a whisker to upgrade the black kat helm to 4*, or use it to make the 3* kat armor?
I would focus my whiskers on the suit, not the helmet. Because at some point you might find a Book of Dark Rituals and get the 5-star helmet immediately. And the 4-star helmet does not upgrade to the 5-star helmet.
Of course, any long-term plan like this is subject to having the developers change the rules (drop chances, item statistics, etc.) midway through.
I wrote a guide on how to choose armor- http://forums.spiralknights.com/en/node/116352
If it's too long for you:
Get chaos and ignore shadow defense lines