So how can I give up knockback or iron will on my totem?

My FT witch just hit Act 4 Merc and notice that my dps is not as effective esp on rares so I would like to swap in Fire Penetration. I can either swap out Iron Will or Knockback. Iron Will gives me like 600 dps out of 5k (prob low no idea) but knockback gives me no dps but a very good mechanism for keeping mobs away from me. Of course, it works best with the mobs that are coming right at me and some slow ones get pushed too far. On the other hand, I can knock Voll into a wall and he can't even do his charge. It's funny stuff.

So if I swapped out Iron Will would Fire Pen make up for 1)its mana cost 2) dps loss from Iron Will?

thanks

Probably. Tooltip won't help, but you should be able to see the difference.