They are very similar, yet different traits. I will give you a couple of examples to illustrate:
Imagine a model that has Quality 4+ and the Battle Hunger trait. In terms of any quality rolls, the model is better than a Quality 4+ model but worse than a Quality 3+ model.
Now, a model that has Quality 4+ and the Will of the Gods trait is a different type of animal. The model is better than a Quality 4+ model but relative to a Quality 3+ model, it has a greater chance to have all failures but also all successes, when making quality rolls on 2 or more dice. Hence, there is greater variance associated with the outcome.
I created Battle Hunger as a racial trait for my orcs, because orcs with combat 3 and quality 4+ alone seemed to weak to me (I come from a warhammer fantasy battles background). Battle hunger makes them closer, yet worse than a quality 3+, which reflects their somewhat undisciplined nature (think Animosity from WFB, if you are familiar with that game).
Will of the Gods was created as I was trying to map the Cult of the Possessed warband from Mordheim to SOBH. Since it increases the chance of multiple failures and multiple successes, it has a very chaotic feel to it.
If you need exact probability distributions, I can send you some. The reason for the existence of these two traits is that otherwise most models are going to be quality 3+ or 4+, without much variety, and I think people will enjoy variety in quality rolls.