I think that the theory is that since you lose some range bonus with the Void, it works best against monsters with a low defense. And from what I can tell, the Iron Dragons have a pretty high defense. That said, I've used both, and I haven't noticed a tremendous difference between the two. Now while the previous post was 95% nonsense, he does make one point that I agree with. Even though I haven't found a big difference between the two, I still wear the void armor simply because it took me so long to get so I want to get some use out of it. If you are in the mood to experiment, you might want to try meleeing them. I tried it a few tasks ago, and it was much more effective than I thought it would be. In fact, the task went by so much quicker, I'm probably not going to go back to ranging them at all.