A little bit of both I guess? It's become standard enough that most web developers (at least most I know) have an account and use it for a few things here and there, it's dirt cheap and widely available, and it's just text files we're storing, so nothing more heavy weight is needed. It does also offer versioning, which I believe we may add a feature to take advantage of for rolling back configuration changes in a coherent way if something goes wrong or to recreate a problem locally on a certain date, but don't think we've started even planning that yet (pull requests welcome!)
And, importantly, has ACL controls built in so it's easy to add/remove access to team members.
Don't forget that its highly durable. When was the last time S3 was down?
Have you had any issues with eventual consistency? Does this need to be run on US-west or EU for read-after-write consistency?
Amazon is not involved in Prism, it seems. So that helps.