Comparing Persona and Google+ Sign In
InformIT just published an article I wrote comparing Google+ SignIn and Mozilla Persona.
TL;DR: If you know you will only care about basic user auth and want a more anonymous experience for users, use Mozilla Persona.
Otherwise, use Google+ SignIn. After the initial bump, it's insanely simple to add more features as Google rolls out more API capabilities both for the user's Google+ account but for general APIs not related directly to auth.
Image src: http://www.flickr.com/photos/mikecogh/6525115985/