Here's a more detailed explanation:
Hundred spears, at max level, has a modifier of [1400% + (1000- weapon weight)] x 1.375 (when you are max level) + 350% if you have lv5 clashing spiral. This is not including the spear boomerang proc.
That means that if you use a glorious spear (0 weight), hundred spears has has 3650% multiplier.
Assuming that you are DB built,
1. A temporal dex boots with EA gives you another 13% range boost.
2. Your FAW EA10 gives you another 20% or 32% (depending on whether you are wearing menblatt).
That would be a total boost of 33% or 45%.
This increases the multiplier of hundred spears to 4855% or 5393%.
Otherwise, if you are using a brocca for multi-purposes (another 20% range boost and def penetration), with the same stats above, the damage will increase to 5841%(menblatt) on mobs.
Downside:
Hundred spears has a goddang 3 seconds global CD.
IB, on the other hand, only gives you a max of 2063% and 3094%(fire) . Using a black ribbon increases the damage to 2681% and 4022%(fire), although in PVP context, there's no way you'd use black ribbon. It also has a same -_-ty global CD, although only 2 seconds.
So yeah, if you're looking for a quick % burst, hundred spears probably does more damage. Problem is both has got -_-ty DPS because of their global cooldowns.