That is a good question and not easy to answer. Generally, there is no big difference (the same for 'sprechen über' and 'sprechen von'), probably 'sprechen/reden über' is used more often. But there is a small difference in the kind of the object. 'sprechen/reden' is used more likely, when we are talking about the characteristics of a person or a thing, e.g. er ist sehr nett, und wir sprechen über ihn. 'sprechen/reden von' is more used when talking about what had happened to him/it. Er hatte einen Unfall. Wir sprechen davon/von ihm (how he is now, what had happened etc.) It is not a really distinctive difference, but it is true that there is a small difference in the perception of the two ways. Yet, there is one case where you can only use 'von'. 'Man spricht von Niveau A1/A2 etc., wenn jemand ....' Here, the word 'das Niveau A1' is explained by the 'wenn'-sentence: It is called ....