@MeuDeus, these two prepositions, along with the word sobre (about), can generally be considered interchangeable with the verb falar, but with some nuances depending on context, in Portugal.
- Sometimes, falar em can imply that the topic of conversation was incidental or brief.
For example: Por falar em filmes, viste o novo lançamento da Marvel? [Speaking of movies, did you watch the latest Marvel release?]
- Falar de is a usual preference when we refer to talking negatively about someone (but this preposition can still be used neutrally anyway).
For example: Porque é que estás a falar dele assim? [Why are you talking about him like that?]
- To refer to the main topic of a conversation/presentation/etc, “falar sobre” and “falar de” are preferred over “falar em”.
For example: Precisamos de falar sobre a Joana. [We need to talk about Joana.]
In the specific example you gave, there’s not a meaningful difference. However, the sentences are not grammatically correct - I suppose you meant Ele falou em si and Ele falou de si 