So I briefly touched on this in the blog post, but to expand a little... ATProto provides significantly more "batteries included" than ActivityPub in my view - if you use ATProto, it can handle both authentication and identity management, and effectively act as your back-end and CRUD operations (eg, oauth with your PDS, and then write/read from the network for your object creation based on your Lexicon).
ActivityPub, based on my understanding, really doesn't work like that - while you an oauth with your mastodon account, the expectation is you'll be handling identity and back-end bits, and then sharing events across the network (happy to be corrected).
Part of what kicked this off is seeing ATProto's new devrel person at a meetup and finding their vision pretty compelling.
But yes, ActivityPub is more "robust" and decentralised (hence also jankier)
https://andreasthinks.me/posts/octosphere/octosphere.html