If you intend to use Cinema for modelling ONLY, you don't need xStream. If you intend to build scenes in Cinema and add Vue atmospheres, ecosystems, terrains, etc, then you need xStream.
Xstream doesn't have more functions/features than Vue Infinite, basically xStream  IS Infinite, but it is Vue Infinite right inside Cinema4D, without the need to import/export, go back to Vue to change settings, re-export to Cinema, etc, etc.
Also consider that ecosystems don't export to any other app.
Cinema 4D's advanced renderer is the fastest around, but the Vue elements will be rendered by the Vue renderer, even within Cinema4d, with Vue 6 xStream, as Peggy said.