It is true that JSON is a lighter format than XML, XML is far more powerful but sometimes it's overkill for a certain purpose. We use JSON internally too to transfer data between server and client.
However when using XML/SOAP Webservices you'd get a WSDL which is basically a contract for the service. You can load this WSDL in the Mendix Business Modeler and it can generate a mapping for it.
This is not possible with a JSON API as it doesn't describe a service explicitly. Using JSON is not and could not be automated in Mendix, you'd always have to define your own mapping. We also don't have a convenient way of constructing JSON requests or handling responses at the moment, if you intend to use JSON here you'll have to implement this using Java actions. It MAY be worth it to do so if performance is a considerable factor here and you have to transfer tons of data across a flimsy line, but I would recommend starting out with webservices.