The difference is that xstream is a plugin to be used in conjunction with other high profile programs like C4d,XSI,Maya,3DS.  In other words, you can work in those programs and render VUE elements into them.  So, open VUE, create a terrain, add an ecosystem, populate it and you can enjoy that element in the other program.  Check e-on's site, they list all the advantages of xstream and a comparison between xstr and inf.