Personally, I use XML with XPath
But yes, json has less overhead over the wire, but, and unsure here, but suspect XML is native to PMS, and json more or less translated from that, before been transferred to the req.
Reason for thinking so, is that all I’ve seen from Plex them self, was using XML
Initially Plex was definitely using XML natively, but if you look at the web app, all communication with Plex is now done over JSON. Yeah the majority of my coding is done with PHP and json_decode($response,true); to turn the data into an array which let’s me use all the native array functions, is just easier than mucking about with trying to parse XML in to an array.