Here's why I think activitypub is probably better.
Having multiple instances, hundreds or even thousands, spreads the load of the network. Smaller instances can curate the communities they want to subscribe to in order to limit traffic and storage. Communities can be hosted across the network too to reduce load on single instances.
This means that when things are done well, we could produce and serve reddit/twitter levels of content and availability on hobbyist level hosting options spread across the world.