Ask HN: JSON-schema or protobuf?

We have been using JSON for our REST APIs and JSON-schema (json-schema.org) to define the data format. We have java and .net generators to create POJO and POCO classes from JSON-schema. Recently, I got to know about Protocol Buffers from Google and was wondering what are the benefits of it over JSON. A simple Google search did not give any satisfactory answer. Hoping to get some insight from people who have used both, better if moved from one to another.