One thing about Nostr is that there is no authentication to retrieve data. So the data can be spread far and wide, without permission.

This can obviously be both good or bad depending on what your use case is!

I think that BlueSky has authentication so that the data only is retrievable on the server specified by the user.

Because of this, I think BlueSky is better for personal social use cases where the user wants to maintain control and feel a bit more safe.