From a practical sense, ActivityPub may be the obvious choice as it gives easier interop with the largest federated platforms.
But what else? There are existing platforms built on these protocols, such as movim for xmpp, and another for matrix I forget.
From a technical standpoint, are there any major pros and cons?
It works better than anything else.
It may seem so due to its success, but I’d argue its success was more tied to luck and timing than technical superiority.
Mastodon was growing and “new” during the period of the decline of social media, and specifically during several moments of Twitter having issues with moderation, and later on acquisition by musk.
Similarly to lemmy, the Reddit third party API fiasco.
I don’t think this means that it works better than anything else. It was just the most obvious choice to users at the time.