Which Should I Use?
If you only want to do authentication and never see yourself wanting to integrate with social services, I’d go with Mozilla Persona. My personal blog is a custom app that I wrote, and the limited scope makes it a good candidate for Persona.
The client-side flow for Google+ Sign-In, on the other hand, is fairly close to the same level of effort. If you are unsure of the final feature set, Sign-In does give you many options of new features you can add. During the writing of these articles, Google added new features to Sign-In several times. The most recent addition was the news that App Activities would be discoverable in Google Search. If I’m running a merchant site, I want all the Buy Activities from my popular products to show up in search where they can influence others to buy.
You could always implement both side by side and warn users that Google+ Sign-In will pull more information from their social profile, making it quicker to populate their account on your website. Google+ is somewhere between 100 and 200 million users strong, but your users need a Google+ account to interact with you. Mozilla Persona, only requiring an email address, which almost every person on the Internet has or can get access to easily, is a much lower barrier to entry.