Pixar's USD was a new level of the open sourced technologies beacause basically we can adopt a high profile pipeline solution originally developed one of the biggest 3D animation studio of the Globe.
MaterialX could be another milestone of the VFX/CGI industry however it is not clear for me how it will evolve. Quote from their description:
"MaterialX provides a schema for describing material networks, shader parameters, texture and material assignments, and color-space associations in a precise, application-independent, and customizable way."
If we look at USD in production viewpoint it is rather a tool for the 3D layout (whatever it means) and of course it is in connection with modelling, animation, fx. So it basically deals with geometries (maybe volumes as well) in a software agnostic way.
But MaterialX propesed to be a standard for look-development data transfer. So it should mean 2D (like textures) and 3D (materials/shaders) type of data in render engine agnostic way. If we think about in a long term it could means every rendering soultion would understand MaterialX data...we will see.
MaterialX was used on the feature film Star Wars Rogue one for example.