i think the difference is subtle, 本質 is the essence of something, so it can be used with both humans and objects/events
e.g. 許多事情的本質其實都是很簡單的,只是人想得太多了。/ 這個人的本質不壞。/ 大自然的本質
whereas 本性 is the nature of a person; it'd be weird to describe something that is without the ability of thinking and feeling as having 本性
e.g. 我的妹妹本性純良,今次不過是受人唆擺。
the more i think about it, the more i think the former is more likely to be the basic and prominent components that form a bigger thing (a man, an event, an object etc.) and the latter is often used to describe the nature of the mind of an individual.