I don’t know the specifics, but ActivityPub has been around longer, so there are larger, stronger communities around compared to the newer kid on the block. You have lots of different services that use the ActivityPub protocol (Mastodon, Lemmy, Pixelfed, Frendica, etc.), all of which have had a lot of time to mature and fix issues, add features, etc. Nostr almost certainly doesn’t have the same yet.

For that reason alone, I would rather use ActivityPub services for now!