Originally Posted by Surgo
Arpen marks seem a bit of a strange choice. You even mention yourself most of his damage is magic (including the early clear damage, because of his W). Even attack speed marks should increase your clear speed more than arpen marks.
They might now since the armor pen rune nerfs. I'm not sure. But while your abilities do magic damage, in clearing the jungle they will take out all the smaller creeps incidentally. The magic doesn't need help. You need to take down the big one, which has armor. And jungle camps don't have much in the way of magic resist to penetrate (read: none). Attack speed could be good, if anything else, but I like armor pen as it helps out with the high armor on the bigger monsters while I don't have a high level taunt to lower the armor and late game when your AD gets decently high.