Honestly, I might not be the best person to ask, as I’m not super familiar with the underlying details of either. But, from a high level, I’d say that Nostr is more decentralized.

(I might get some of these details wrong, so hopefully someone can correct me.)

I’ve seen people say that ActivityPub is decentralized, but I just don’t quite see it. I think even email is more “decentralized” than AP. With email, your identity is tied to a domain name. If you own the domain name, then you can switch email servers/providers. AFAIK, with AP, your identity is tied to a server and domain name. So if the server is gone, then your account is too.

With Nostr, you own your identity. It’s not tied to a domain name or server. If a Nostr relay goes offline (or bans you, I suppose), you can continue using your account through other relays.

I think most people use more than one relay on Nostr (that’s the default setting in many Nostr clients). If one goes down, I’m not sure I’d even notice. You can add/remove relays from your client too.

Nostr feels a bit more like p2p, in a way. More of the heavy lifting is done on the client than the servers/relays.