How they are different is that you get something other than a ringback tone when you place a call. Something such as coupons, somebody you follow social media posts, or even music. Apparently, they are finished with their beta testing and plan to launch soon. No information is on their website other than requesting your email if interested.
RingPlus today announced that they have signed an agreement with Sprint. Furthermore, they plan to launch q3 in New York, San Francisco, and Los Angeles. Sounds like they might be sending you local ads thus limiting users to people who live in those markets.
Sounds like you can customize the ringback for news or music which probably includes ads as well. Maybe for the zero bill thing they mean that one day there will be enough users & ad revenue so that there are no user fees. Seems a little optimistic in my opinion.